FILICINEM. 481 



formation of the tapetum, the remaining cells being spore-mother-cells. The tapetal 

 cells then divide by transverse and longitudinal walls, so that the spore-mother-cells 

 come to lie deeply within the tissue of the sporangium. 



In the macrosporangium of Isoetes each spore-mother-cell divides to form four 

 macrospores ; its nucleus divides into two and each of these again into two, before 

 any intervening cell-wall is formed. There is this peculiarity about the mode of the 

 division, that the protoplasm, as in the delevopment of the spores of Aiilhoceros, 

 begins to divide before the nucleus. The spore-mother-cells of the microsporangium 

 divide in a different manner, the only other known instance of the kind occurring in 

 the pollen-mother-cells of Monocotyledons \ In them the nucleus divides into two, 

 and this is followed by the formation of a cellulose wall between the two cells : the 

 nucleus of each of these then divides, and a wall is formed between the resulting 

 cells. It is in this way that the four ' special ' mother-cells of the microspores 

 are produced.] 



Fig. 339.— a transverse section of the stem ol Selagiiiella denticiilata, the central vessels of the bundle not yet lignified; 

 b air-cavity surrounding a bundle which is being given off to a leaf. 



Histology'^. In the Selaginelleae, to which group the following remarks more 

 especially apply, the epidermis of the stem consists of long prosenchymatous cells 

 between which no stomata occur. The cells of the epidermis have often beautifully 

 sinuous lateral walls, and, like those of the Ferns, they contain chlorophyll which 

 occurs in these cells as well as in the cells of the fundamental tissue of the leaf in the 

 form of large granules, only a few of which are to be found in each cell (Fig. 44). The 

 leaves usually possess stomata on the under surface only, but they occur on both surfaces 

 of the small leaves of S. pubescens. In several species (such as S. stenophylla and Martensii) 

 single epidermic cells occur with walls so thickened that the lumen is almost occluded. 

 (Russow). In most of the species the epidermis of the upper differs from that of the 

 under surface, in others {S. Gakotti, Kraussiana) the epidermis of the two surfaces is 

 of the same nature. 



The Fundamental Tissue of the stem consists, as \r\ Lycopodium, of elongated cells 

 with septa which are either oblique or transverse: these cells retain, however, their 

 thin walls and large cavities, in contrast to what is usually the case in the Lycopodieae, 

 the hypodermal layers only becoming thick-walled (Fig. 340). It appears that the cells 

 of the fundamental tissue, and consequently those of the other tissues also, are capable 



' [Strasburger, Zellbildung und Zelltheilung, 3rd ed., 1880, p. 167.] 

 ^ [For further details see De Bary, Vergleichende Anatomic, 1877.] 



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