414 PART III. THE CLASSIFICATION OF PLANTS. 



sporons leptosporangiate Ferns in all essential features of their form 

 and development, as also in their extrusion from the aritheridium. 

 In the Marsileacese, the male prothallium is enclosed within the 

 microspore until the antheridium is mature, when the spore- walls 

 are ruptured by the swelling of the cells of the contained pro- 

 thallium, and the spermatozoids are set free. 



The female individual is a small mnlticellular prothallium of a 

 green colour, and is developed from a macrospore to which it re- 

 mains attached. The development of the prothallium begins 

 inside the macrospore at its pointed apical end, where there is 

 an aggregation of protoplasm in which the nucleus lies. The 

 nucleus divides, and this is followed by the formation of a cell- 

 wall between the two nuclei, cutting off the apical portion of the 

 spore, as a small cell, from the basal larger portion ; this first 

 wall is termed the diaphragm, and it marks off that portion of the 

 macrospore which gives rise to the prothallium from that portion 

 which takes no part in the process. The small cell then under- 

 goes repeated division to form the prothallium which consists, in 

 the Salviniaceae, of several layers of cells (at least in the middle 

 region), and in the Marsileaceae of two layers only. As the pro- 

 thallium developes, the coats of the macrospore split into three 

 valves at the apex, so that now the prothallium i in direct rela- 

 tion with the exterior. Whilst in the Marsile$,cea3 the prothallium 

 projects but little from the spore, in the Salviniaceae (especially 

 Salvinia), where it is larger, the greater part of it is outside. In 

 Azolla its outline is that of a rounded triangle, projecting between 

 the lobes of the epispore, and throwing off the cap formed by the 

 wall of the sporangium and the indusium ; in Salvinia it is more 

 acutely triangular, two of the corners (anterior) growing out into 

 long wing-like appendages, and breaking through the wall of the 

 sporangium. 



There is no clear differentiation of an archegoniophore, such as 

 occurs in some of the homosporous Leptosporangiatae (see p. 401), 

 though there is an indication of this in Salvinia where the pro- 

 thallium projects somewhat in the median line, especially an- 

 teriorly, where the archegonia are borne. 



No cell-formation takes place in the larger basal portion of the 

 macrospore below the diaphragm, though nuclear division has 

 been observed in Azolla. This eventually becomes filled with 

 starch and other nutritive substances for the nutrition of the 

 embryo. 



