CYTOLOGY OF REPRODUCTION IN PLANTS 



157 



The zygote upon germination produces four nonmotile or motile spores 

 with the reduced chromosome number. These in turn develop into 

 new plants. 



Oedogoniiim produces motile asexual zoospores and male gametes 

 which resemble them in having a crown of many cilia (Fig. 115). The 

 female gamete, how^ever, is a large nonmotile egg with a visibly differ- 

 entiated "receptive spot" at the point where the 

 sperm is to enter. The nucleus lies near this spot. 

 The sperm passes through a pore in the wall of the 

 cell bearing the egg, and syngamy takes place. 

 The resulting zygote ripens into a resting oospore, 

 and as it does so meiosis occurs. Upon germina- 

 tion of the oospore four zoospores, each with the 

 reduced chromosome number, are produced, and 

 these develop into new plants. Some species are 

 heterothallic, a segregation of the two sex ten- 

 dencies occurring at meiosis. 



The motor apparatus of the motile cells of 

 Oedogonium is noteworthy (Fig. 116). The 

 nucleus of the cell which is to transform into a 

 zoospore comes in contact with the cell mem- 

 brane, which there forms a convex thickening. 

 A ring of granules appears in this region, and as 

 the nucleus moves away the ring becomes double, 

 a crown of cilia then growing out from the outer 

 half of the ring. Such cilia-bearing organs formed 

 apparently by the cell membrane are called 

 plasmodermal blepharoplasts to distinguish them 

 from the centrosomal blepharoplasts of ferns and 

 mosses. It is of interest to observe that the 

 zoospore resembles the motile gamete morphologi- 

 cally^ in Ulothrix and Oedogonium, although the 

 morphology differs in the two genera. This sug- 

 gests a common origin of the two types of reproductive cell. 



Syirogyra differs sharply from the genera described above in having 

 neither zoospores nor other ciliate cells. At the time of sexual reproduc- 

 tion two cells become joined by a conjugating tube (Fig. 117). Their 

 two protoplasts with their remarkable chloroplasts then become slightly 

 modified in appearance and behave as gametes. One of them may 

 pass through the conjugating tube to the other, with which it then 

 unites, or both protoplasts may move into the tube and unite there. 

 Contractile vacuoles have been found to play a role in the movement of 

 these gametes ])y withdrawing water from the central sap vacuole and 



Fiu. 115. — Sexual re- 

 production in Oedogo- 

 nium. One cell of the 

 filament has enlarged as 

 an oogonium and contains 

 a large egg. A sperm is 

 about to enter the oogo- 

 nium by way of a pore 

 with an extruded slime 

 papilla. {Modified from 

 H. Khhahn.) 



