Portulaca oleracea: Seed Structure

פורסם: March 28th, 2010 | עודכן: 17/01/15

When I started writing the special plants chapter, I thought I would deal with trees first: the “hyssops,” included by King Solomon the wise as a type of tree. After completing that chapter and another one, I dealt with determination of seeds of the [“Portulaca oleracea” P. oleracea] group, collected by my colleagues in Sardinia, Italy, during the summer. This is part of an ongoing project in which I found a species new to science which will be named after Sardinia. Additional findings urged me to return and work intensively on this group of plants, which I have been studying since 1977.

In order to understand the way I investigated them, I should mention all my involvement with the group. While writing the Hebrew version I received some 700 seeds derived from archaeological investigations in Italy. I deliberated for a long time about how to construct and present this chapter. Inserting all the micro-species (The Organization of Phylo-Taxonomic Investigation of the Mediterranean Area – OPTIMA – uses the term “Micro-species” for taxonomic units that are hard to define, at the present stage of investigation, as being at the level of species, subspecies, or variety) derived from the recent taxonomy of the group forced me to provide a “key” in Hebrew for their determination; such a key was published in our article [https://flora.org.il/en/articles/microspecies-of-the-portulaca-oleracea-aggregate-found-on-major-mediterranean-islands-sicily-cyprus-crete-rhodes/ Danin, Domina & Raimondo (2008).]).

Seed structure of the [“Portulaca oleracea” P. oleracea] aggregate and terms used

To examine the seeds one has to use light from a matt bulb providing dispersed light. Regular light derived from a clear bulb makes the seed surface shiny and interferes with the observation of details of the epidermal cells morphology. I normally start my observations using 10X magnification and then change to 400X. A seed of [Portulaca oleracea] (in the narrow sense) is displayed in Fig. 11.1.1. [Portulaca] belongs to the Centrospermae, an order typified by a coil-shaped embryo, and one can certainly see one end of the coil near the figures 907.57. The line, terminated by the above number, marks the long diameter (almost 0.91 mm). A perpendicular line extends along the short diameter of this elliptic body. We are going to use here only the longer diameter. The tip of the embryonic root is situated below the seed coat, at the left side of the line. The embryo is coiled, and the tip of the pair of embryonic cotyledons is below the “net with small squares.” The “net” is a scar found in each seed as part of the funicle (the minute elongated organ used to feed the ovule-seed), which was connected to the ovary. The lines forming the “net” are remnants of its cells.

In order to display the embryo here, I saturated [Portulaca] seeds with water and carefully opened two seeds at different stages of saturation. A coiled embryo surrounding a small food cylinder (endosperm) is seen in Fig. 11.1.2. The upper part of the embryo terminates towards the right, by the embryonic root cap. In the embryo magnification (in Fig. 11.1.3) the “net” of cells discussed above is seen as well. The embryo in Fig. 11.1.4 is at a more advanced stage of development. Its cotyledons are prominent whereas the root tip is hidden below remnants of the seed coat. The cylinder at the center of the embryo is the endosperm (it is worth reading about it in a classical Plant Anatomy such as that of A. Fahn); the round wall between the endosperm and the seedling is the seed-coat.

Epidermal cells star-shaped; arms with no emergences

Fig. 11.1.1: Portulaca oleracea, a seed.

Fig. 11.1.2: A coiled embryo.

Fig. 11.1.3: The tip of the embryo root and the cotyledons.

Fig. 11.1.4: A water-soaked embryo with a pair of cotyledons, endosperm and seed coat.

Epidermal cells of the lateral sides of the seed differ from taxon to taxon in their morphology and are used as diagnostic characters. The “stars” in (Fig. 11.1.5) are densely packed epidermal cells. The connection between the cells is so tight that seeds found in archaeological excavations in Italy, after 1000-1500 years of burial, broke without any relation to the break lines and the cells’ sutures (Fig. 11.1.6). The cells may be “isodiametric”; one may imagine all the parts of one cell as nestling within a circle (Figs. 11.1.7a, 11.1.7b) or “elongated” with an ellipse encircling its parts (Fig. 11.1.7c, Fig. 11.1.7d). “Fingers” project from the main body of the cell and fit very accurately with the “fingers” of the neighboring cells. The “fingers” may be long (Fig. 11.1.7b) or short (Fig. 11.1.7c) with small protruberances (Fig. 11.1.8) which comply with the scientific term “granulate” (meaning grained). The height and diameter of these emergences may be measured using a scanning electron microscope (SEM), a tool people do not normally keep in their pockets. The SEM of the Geological Institute in Jerusalem, or of The Hebrew University fills up a special research room. The “finger’s” back may be smooth; no grains are seen on the “fingers” of Figs 11.1.7a, 11.1.7b and 11.1.7d. The central part of the cell may carry a tubercle such as in [http://flora.huji.ac.il/browse.asp?lang=en&action=specie&specie=PORCYP Portulaca cypria] (Fig. 11.1.9) or two to three tubercles on that of [“Portulaca trituberculata” P. trituberculata] (Fig. 11.1.9). There is a group of three microspecies which have relatively small tubercles near “grains” (Fig. 11.1.11, 11.1.12). The diagnostic characteristic used within this group is seed diameter. In a very high enlargement of seed surface (Fig. 11.1.13) there are properties that may be utilized for differentiation in special cases. Wax covers the surface of one microspecies (P. nicaraguensis – Fig. 11.1.13b). In [http://flora.huji.ac.il/browse.asp?lang=en&action=specie&specie=PORCYP P. cypria] (Fig. 11.1.13d) one can see swellings that resemble human skin at the site of a mosquito-bite. There are species with a smooth seed surface (Fig. 11.1.13a).

Fig. 11.1.5: Portulaca oleracea, seed-coat epidermal cells.

Fig. 11.1.6: Portulaca cypria from an archaeological site in Italy. Seed-coat breaks that do not follow the sutures among cells.

Fig. 11.1.7: Epidernal cells’ morphology: a. Top left – isodiametric cells of Portulaca cypria, b. Top right – isodiametric cells of Portulaca nitida, c. Lower left – elongated cells of Portulaca canariensis, d. Lower right – Portulaca nicaraguensis.

Fig. 11.1.8: Stars and “grains” or papillae, Portulaca granulatostellulata.

Fig. 11.1.9: Tubercle at the center of each cell, Portulaca cypria.

Fig. 11.1.9b

Fig. 11.1.10: Three tubercles in the seed epidermis cells, Portulaca trituberculata.

Fig. 11.1.10b

Fig. 11.1.11: Tubercle and papillae of a similar size, Portulaca rausii.

Fig. 11.1.12: Tubercle and papillae of a similar size, Portulaca zaffranii.

Fig. 11.1.13: Types of epidermal cells surface.

And what about leaves, stems and flowers?

Fig. 11.1.14: A typical adult plant of the Portulaca oleracea aggregate. Accurate determination is possible only by seed examination.

No link may be seen between morphological properties when the green canopy of the plant and its flowers (Fig. 11.1.14) are studied. Plants with the same, or similar seed morphology may have short or long petals or may even be without any petals (cleistogamous). Stems of plants with similar seeds may be whitish-green near the reddish stems of their neighbors. There is no constancy in the link between the morphology of vegetative parts in microspecies recognized by their seed morphology. Raising plants from seeds collected in the field gave rise to offspring with similar seeds to those of their mother-plants.