148 



MANUAL OF BOTANY 



>a 



through the wall of the sporangium. It then forms a septum at 

 the end, cutting off a terminal cell which later divides into two. 

 The tube constitutes the prothallium, and its two end cells after 

 further divisions form a rudimentary antheridium, in which are 

 produced four antherozoids {fig- 906). 



\¥henthe antherozoids are mature, the antheridium ruptures 

 as in other cases, and the antherozoids, each in its mother-cell, 

 escape into the water. Only part of the protoplasm of the 

 mother-cell is used in the formation of the antherozoid, as in the 

 ferns. 



AzoUa produces a similar male gametophyte. In the Marsi- 



leacese the latter' is 

 Pis- 906. formed within the 



microspore. 'The first 

 division of the spore 

 produces a small basal 

 vegetative cell and a 

 larger apical one, 

 which forms an anthe- 

 ridium. By successive 

 divisions this comes to 

 consist of eight cells 

 surrounded by a pa- 

 rietal layer or wall. 

 The central cells pro- 

 duce each four anthero^ 

 zoids. Throughout the 

 group the male pro- 

 thallium is destitute of 

 ohloroplastids. 



The formation of the female prothallium is a good deal 

 alike in all the genera, showing small differences in the extent 

 of its protrusion from the spore, from which it is never free. 

 The macrospore begins to germinate before its coats rupture ; it 

 first cuts off a small cell at its anterior end or apex, by a wall 

 known as the diaphragm, which thus divides the spore into 

 two. The small cell at the apex continues to divide, forming a 

 small celled tissue which soon protrudes through the spore- 

 coats, owing to the rupture of the latter. The emerging tissue 

 develops chloroplastids and becomes green ; it constitutes the 

 prothallium. In Salvinia it is somewhat triangular in shape and 

 bears two winged appendages (fig. 007). In the Marsileaces only 

 a small part of it protrudes from the opening of the spore. 



Fig. 906. Germination of mici'ospores of Salcinia. 

 After Sachs. 1. The mass of spores putting out 

 tubular prpthalli. 2. A prothallua, with anthe- 

 ridium, a, 3. Antlierozoids in mother-cells. 

 4. Ruptured antheridium. 



