INTRODUCTION

  My very small tropical garden is especially composed of leafy plants (plants which seldom flower but which show very attractive foliage forms and colors, like Epipremnum , Syngonium , Chlorophytum, Codiaeum, or "the Water wood" (Dracaena fragans), Tradescantia (and many others) and by small flowering plants (Portulaca (2 sps), Aptenia , Hybiscus, "alegrias", Kalanchoe). These last are, moreover, plants which can reproduce from cuttings, of very fast growth and of easy dissemination. Portulaca oleracea , Aptenia, and Syngonium podophyllum, Epipremnum aureum or Chlorophytum comosum too, are very attractive to be used in flowerpots with hanging foliage which are very much used to give greenery and freshness inside and outside of our houses.

  Using a search engine you can find pictures of all of these on many garden or botanical sites on the Internet.

  Few of them (even the most characteristic and abundant in the local gardens) are autochthonous plants.

  Aptenia cordifolia (Lf) Schwante, which I present here, is a modest trailing plant, of waxy appearance, soft consistency and with small red flowers. It was transferred to Europe from an area near Johannesburg and subsequently dispersed in the Americas , starting with an importation made to the United States in 1970 from Israel .

 It is very common in Mexico , and I used it in Durango to obtain epithelium samples (which have a very interesting morphology, and are very easy to peel from the leaf underside) to be used in my work on the mounting media.

  In Cancún small cuttings planted in sunny places, without any greater precaution, produce dense plants full of flowers in a few weeks. If one doesn’t wish that it becomes a plague, its growth must be carefully controlled, because it outgrows any other plant which appears in its way.

  Its best destiny is to be cultivated in flower-pots.

The title picture shows the foliage and the flowers of a young plant.

 We will see the more important macro and microscopic features of this plant in two sections: 1) vegetative organs and 2) reproductive organs.

VEGETATIVE ORGANS

THE LEAF. The following pictures show the epithelium of the LEAF at three different powers. Leaves are hypostomates that is to say that they have the stomata in the lower or ventral face of the leaf.

As one can see very well in the picture captured with the 100x objective, the stomata are simple, both guard cells being surrounded by normal cells of the epithelium. On the contrary, composed stomata, like those of Tradescantia, are surrounded by additional cells different from the normal epithelial cells, although they also do not have chlorophyll. (See a picture, here)

  There is a special detail which I saw in the epithelium of no other plant, either those on which I made a direct study, or in the images that I have seen.

 They are those which I call lacunar cells, very large, much thickened, with an internal enormous vacuole and a thin peripheral cytoplasm, which has a very large nucleus, generally on the external layer of the cytoplasm. These cells form a very characteristic network, as can be appreciated in the surface image taken with the 10x objective.

Epithelium surface -x10 obj	Idem - x40 obj.	idem - x100 obj

  The foliar epithelium is the only part of this plant which permits easy research. Leaves and young stems have a very soft consistency and a loose structure, in which the cells appear to float in a mucilaginous medium. To obtain clean sections, which also cannot be too thin, the edges of the mesotome must be much sharpened.   For better success it is well to let materials desiccate for 2 or 3 hours. One obtains a much firmer texture.

  Another particular characteristic of the leaves is that at the start I could not identify a vein or a definite network of veins. Like all the leaves, those of Aptenia have, as it can be seen in the first picture, the shape of a butterfly with a central gutter which makes us suppose the existence of a main vein. However as soon as I carried out one transverse section of the leaf crossing the "gutter", I have been able to see only one continuous blade which, even under the objective, lost its 'butterfly' shape very quickly.

Some randomly oriented vessels were identified where the central vein of other leaves are clearly seen. Compare with other leaf sections in my previous papers.

One can find a central vein only in the posterior third of the leaf but with a weak contrast which makes difficult its identification.

5 - Rib, in the proximal third of the leaf. A vascular package. A mosaic of two images.

6 - distal third of the leaf without the vascular bundle structure. Two stitched pictures.

And the vessels, which moreover are numerous, are cut in small sections, as if they had a very sinuous path, without a well defined direction. In the following stitched pictures (figs. 7 and 8) the vessels are the central, longitudinal, dense structures (dark in the brightfield image; and bright in the   darkfield one). The small whitish spots distributed in the parenchyma are clusters of raphides.

Fig. 7 - Transverse section (in anatomical position: underside downwards) x10 obj.	Fig. 8 - similar cut. In the underside one appreciates a large lacunar cell (arrowed).

  In the sections of the leaf blade one can define the two layers of epithelium (higher and lower). In the lower epithelium the large and long lacunar cells are easily differentiable.

9 - 3 lacunar cells, x10 obj.	10 - Rhaphides (in red) in the parenchyma, x10 obj.

An image of a lacunar cell with a higher power is shown in the following brightfield picture (fig 11). 

x40 obj.

  Searching for the "veins". In order to be able to see the distribution of veins in the leaf mesophyll I used the following technique.

a) Washing out the chlorophyl .- 2 leaves were boiled in water for 10 minutes in order to well soften the cellular walls, and they were then boiled 20 min. (80ºC) in 96% alcohol in order to eliminate chlorophyll. The process bleaches leaves completely.

  b) Clarification.- To see through them they could have been impregnated with glycerin (IR=1.47). As I did not have it at the time, I used castor oil (IR=1.48) that it is also miscible with alcohol 96. After 3 hours of contact, the leaves are transparent. One of them was mounted between slide and coverslip, but as it was curly I pressed it with 4 weights of 15g for 14 hours.

12 - Epithelium and sub epithelium cut longitudinally, x40 obj.	13 - one vascular vein with spiraled-wall vessels
14 - Aptenia leaf  - network of veins - X 4	15 - idem 14, x 10 obj.

c)  Seeing through the leaves shows precisely what I suspected. In the greatest part of the leaf the vessels describe a sinuous form with the result that a right cut shows them like separate segments. And they form a network (which, curiously, one does not see as completely connected), which covers all the leaf by forming a fine mesh, but not a main vein as in most leaves.

     d)  They are less easy to see the sections of the stomata, and one must look between the cells of the lower epithelium of the leaf. But in the following figure, one sees the two guard cells charged with chlorophyll and under them a pear-shaped air cavity, the stomatic cavity (fig.16).
 

Fig. 16 - Stoma from the side X100 - on the underside epithelium. Pear-shaped air cavity, the stomatic chamber.	Fig. 17 - Parenchyma with aspect of palisade

There is a sub-epithelial layer of 2 or 3 small cells, globose, charged with chlorophyll (very visible in darkfield) and below, or intermingled with them, a layer with large rectangular cells, also with chlorophyll, which appear to represent the palisade epithelium of more conventional leaves (fig. 17 and 19).

 The two or three following layers represent a mesophyll with globose cells, between which there are an abundance of idioblasts with acicular raphides of calcium oxalate (fig.18).
 

Fig. 18 - acicular calcium oxalate in a parenchyma cell	Fig. 19 - palisade-like parenchyma

FOLIAR PETIOLE. The petiole of the leaf is more difficult to interpret. It shares the same loose structure of the leaf, although it has a more defined indicator of a principal vein (fig. 20). The epithelium now takes the special form which we will study in detail in the stem. The cross sections must be observed quickly to prevent them completely losing their structure in a few seconds.

Fig 20 - Cross section of the petiole - a set of 13 individual pictures amalgamated The original image has 1685 px x 1801 px.

 
THE YOUNG STEM. Transverse Section – The stem has a square section which becomes more defined as the stem aged. The image shown in fig. 21 is from a young stem and was made by stitching 8 individual pictures.
 

Fig. 21 - Cross section of the stem of Aptenia. Original pictures taken with the 10x objective. Stitched in PhotoPaint. Background replaced in the photoprocessor.

Pictures were taken using a Rheinberg filter with crimson center and greenish outer ring. Its structure corresponds obviously to that of a dicotyledon. From the outside towards the interior one can initially describe an epithelium which is very special in many ways. Contrary to what the epitheliums of other plants that I examined are, which generally show its protective function by the thickened external cuticle, often covered of wax, and by the small and thin cells which conform them, this epithelium of Aptenia is formed by long and high cells, of round external profile with the shape of a cushion (with rectangular plant, which in the transverse section are seen like transparent globules of fine cellular wall) (figs.22 and 23).

Fig. 22 Sector of a cross section. Obj. X 10 - RhA/Am	Fig. 23 Cortical parenchyma and skin of the stem in cross section. Obj. X 100

To recognize its true form it is necessary to examine the longitudinal section of stem which we present in fig. 29.

  Under the epithelium there is a sub-epithelium with 2 or 3 layers of cells with chloroplasts, and then we see a cortical parenchyma.

  Dispersed inside the cortical parenchyma one finds several small packages with some fibers, and few small vessels of xylem (2, even 4 - Fig. 24), in addition to many crystals of calcium oxalate. The vascular packages in the cortical parenchyma are atypical in a dicotyledon, but they are clearly recognizable (but difficult to photograph - they are very dense and obscure). They generally have two or three woody vessels of very small diameter.
 

Fig. 24 Four fibers bundles, with some vessels, found in the cortex

The cortex is limited internally by a layer of cambium (refringent yellowish lines, in figs. 21 and 22) which surrounds the central cylinder within the limit of which one finds the principal vascular packages, with a very specific design.

  At the two sides of the central cylinder one finds two vascular bundles provided with phloem, xylem and fibers, in definite layers. (figs. 27 and 28)

Fig. 25 - Cortex - chlorophyllian parenchyma, packages of fibres (large diffuse spots) and crystals, (small diffuse spots ).	Fig. 26- A similar image in brightfield. Moreover one sees in the cambium two secondary vascular packages. See also the epithelial cells in transversal section.
Fig. 27 - One of the two principal vascular bundles, in brightfield. The crystals appear as dark heavy points.	Fig. 28 - COL-D3 filter better defines the complex tissue structure of the principal package.

Moreover they are between these large packages, located in equidistant positions, 5 small sized bundles in one semicircle, and other 5 in the opposed one. (figs.-  21)

  Pith parenchyma has large globular cells which also show drusses.

 
I made a longitudinal section of the stem, which the mesotome allows to be do.

  Line A-B drawn through the transverse section in fig. 21 shows a transect which could provide a longitudinal section similar to that really made. To show the most important anatomical details we will use a rebuilding of the section prepared with 4 individual pictures, taken with the X10 objective (fig. 29).

Fig. 29 – A band of the longitudinal section of stem. 5 pictures joined in PhotoPaint (X10). The band has a height of 640 pixels, equivalent to 1333 microns. Click on the picture to see the original mosaic

A t the two sides of the section we confirms the globose and aqueous nature of the epithelial cells which now shows their long vertical dimension, at least 3 times larger than the transverse diameter. With the chosen power one can only roughly define the characteristics of various tissues although is quite visible in its topographic distribution and its nature.

 But in the stitched band they are quite visible the details which we have described in the coss section: the epithelium followed inside by a chlorophyllian sub-epithelium, the cortex, in which one appreciates the chalky lines of cortical fiber beams (complete at right image, incomplete on the left, surely because it was cut obliquely), and the parenchymatic cells themselves with loose and aqueous aspect.

  Details below enable us to better describe their histology.

Pictures 30 to 33 show aspects of the cortex

Fig. 30 - Cortex - one can see two idioblasts charged with raphides and red  colored by the filter (W-Q, something displaced)	Fig. 31 - Transition of cortex to pith. Cortex with a small package of fibers (downwards) cambium (coffee), light blue phloem and vessels of the orange xylème. W-Q
Fig. 32 - idioblasts of Fig. 30 in brightfield (x40)	Fig. 33 - two crystals in the cambium. On the left one sees fibers and a part of a woody vessel. Brightfield

and the following ones (34 and 35) details of the vessels of a main bundle :

Fig. 34 - spiral and tracheid vessels	Fig. 35 - tracheids

THE OLD STEM.- With age the stem becomes larger and harder, although it does not exceed a diameter of 5 or 6 mms. A large schlerenchyma layer is formed out of the cambium circle, and the whole resembles much the structure of the old stem. An image of the latter was included in the article on......

A sector of a cross section that shows from the pith parenchyma to the phloem is included here.
 

36 - See the difference in thickness of the woody sector and of the cambium sector by comparison with figs. 25 -28. The section shows from the pith (at right) to the cortical parenchyma (at left).  X40 Two assembled pictures.