430 



PART III. THE CLASSIFICATION OF PLANTS. 



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 

 (p. 14). 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. 



THE GAMETOPHYTE. 



Selagiuella being heterosporous, the gametophyte-generation is represented 

 by distinct male and female individuals, which are rudimentary prothallia 

 bearing the male and female organs respectively. 



The male prothallium is developed inside the microspore : the germination 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 enclos- 

 ing the mother-cells of the spermatozoids. When the development of the 

 spermatozoids is completed, the coats of the microspore burst, as also the wall 

 of the antheridium, and the spermatozoids are set free. 



The male cells or spermatozoids consist of a somewhat club-shaped slightly 

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



The female prothallium is developed inside the macrospore (Fig. 278) : ger- 

 mination 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 con- 

 sisting 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 (sometimes called endosperm) being produced 

 which serves to nourish the embryo -which is forced down into it by the elon- 

 gation 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 becomes green, is exposed. 



The female organ or archegonium is developed from a single superficial cell 

 at the centre of the upper surface of the prothallium (here several cells thick) ; 

 it does not call for any special description : if the first archegonium fails to be- 

 come fertilised, others may be subsequently formed. 



The female cell or oosphere is contained in the venter of the archegonium. 



