GROUP III. PTERIDOPHYTA : LYCOPODIN^E. 393 



small-celled stratified meristem. The structure of the growing-point of 

 the rhizophore agrees with that of the stem in the various species : but 

 the apical cell, when present, is a four-sided pyramid at first, becoming 

 eventually two-sided. The growing-point of the root has a tetrahedral 

 apical cell. 



Embryogeny of the Sporophyte (p. 347). The embryogeny of Selaginella 

 closely resembles that of Lycopodium. The oospore undergoes division, a 

 transverse basal wall being formed : the upper or hypobasal cell developes 

 into a unicellular or few-celled suspensor : the lower or epibasal cell appears 

 to undergo division into four octants, which eventually form two tiers of 

 cells : from the basal tier of cells the hypocotyl is developed ; from the 

 apical tier the growing-point of the stem and those of the two cotyledons. 

 The hypocotyl elongates, and curves so as to escape from the prothallium 

 and the spore ; the convexity of the curve becomes somewhat protuberant 

 and is usually (but erroneously) termed the " foot," though it doubtless 

 acts as an organ of absorption ; morphologically it cannot be a foot since 

 it is epibasal in origin ; it would more appropriately be termed a feeder. 

 The first root eventually springs, endogenously and adventitiously, from 

 the posterior portion of the convex surface of the hypocotyl ; it is not a 

 true primary root because it is epibasal in origin (Figs. 233, 234). 



THE GAMETOPHYTE. 



Selaginella being heterosporous, the gametophyte-generation is repre- 

 sented by distinct male and female individuals, which are rudimentary 

 prothallia bearing the male and female organs respectively : the sexual 

 organisms resemble those of Isoetes (p. 357) and of Gymnosperms. 



The male prothallium is developed inside the microspore : the germina- 

 tion of the spore begins with the formation of a wall across the pointed 

 apical end of the spore, cutting off a small cell, the vegetative cell : the rest 

 of the spore goes to form the single antheridium which consists of a layer 

 of parietal cells enclosing the mother-cells of the spermatozoids. When 

 the development of the spermatozoids is completed, the coats of the micro- 

 spore burst, as also the wall of the antheridium. and the spermatozoids are 

 set free. 



The male cell is a spermatozoid ; it is a somewhat club-shaped slightly 

 twisted body, bearing two cilia at its pointed anterior end. 



The female prothallium is developed inside the macrospore (Fig. 233) : 

 germination begins with the formation of a wall, termed the diaphragm, 

 across the apical end of the macrospore : in the smaller upper cell thus 

 cut off cell-division proceeds, resulting in the formation of the meniscus- 

 shaped prothallium consisting of compact small-celled tissue : the larger 

 portion of the spore, below the diaphragm, is rich in reserve materials: 

 here cell-formation goes on but slowly, a large-celled loose tissue (some- 

 times called endosperm) being produced which serves to nourish the em- 

 bryo which is forced down into it by the elongation of the suspensor. 



The walls of the spore eventually split along the angles, thus forming 

 an apical aperture by means of which the upper surface of the prothallium 

 which now oecomes green, is exposed. 



