XII LEPTOSPORANGIATyE HETEROSPORE^Z 407 



time, and in its later stages develops abundant chlorophyll, 

 and its basal superficial cells grow out into colourless rhizoids. 

 In case the archegonium is not fertilised, the prothallium grows 

 for a long time, and reaches considerable size, but never 

 develops any secondary archegonia. In Pilularia, both 

 prothallium and embryo may develop chlorophyll in perfect 

 darkness.^ 



TJie Embijo ^ 



The two genera correspond very closely in the development 

 of the embryo, which shows the greatest resemblance to the 

 Polypodiaceae. In Marsilia the development of the embryo 

 proceeds very rapidly. The first division of the (t<g<g is com- 

 pleted within about an hour after the spermatozoid enters, 

 and in Pihdaria after about three hours, as nearly as could 

 be made out. In both the basal wall is vertical and divides 

 the somewhat flattened &gg exactly as in Onoclea. The quad- 

 rant walls next follow, and then the octant walls, as usual. 

 Of the latter the one in the root quadrant diverges very strongly 

 from the median line (Fig. 212, C), and that in the foot quad- 

 rant is much like it. In the others it is nearly or quite median, 

 and it is impossible to say which of the leaf and stem octants 

 is to form the apical cell of those organs. The relative posi- 

 tion of the young organs is exactly the same, both with 

 reference to each other and to the archegonium, as in the 

 Polypodiaceae. 



TJie Cotyledon 



The cotyledon grows for a time from the regular divisions 

 of one or both of the primary octant cells, but this does not 

 usually continue long, and the subsequent growth is purely 

 basal. The cotyledon is alike in both genera, and is a slender 

 cylindrical leaf tapering to a fine point, where the cells are 

 much elongated and almost colourless. Its growth is at first 

 slow, but at a later period (in Pihdaria glolndifera about the 

 eighth day) it begins to grow with great rapidity and soon 

 reaches its full size. This is largely due to a simple elongation 

 and expansion of the cells, which are separated in places, and 

 form a series of longitudinal air-channels separated by radiating 

 plates of tissue (Fig. 213, i). The simple vascular bundle 



^ Arcangeli (l), p. 336. - Hanstein (2). 



