114 



INTRODUCTION TO CYTOLOGY 



contains a single plastid which divides with the nucleus at each cell- 

 division. The egg likewise contains a plastid, but the spermatozoid has 

 none: the fertilized egg and sporophyte cells which it later forms are 

 therefore characterized, like the cells of the gametophyte, by the presence 

 of one plastid. Although it is difficult to demonstrate the plastid in the 

 young sporogenous cells, every sporocyte shows one very clearly. As 

 shown by Davis (Fig. 42), the sporocyte plastid divides twice during the 

 prophases of the first (heterotypic) division of the sporocyte nucleus, so 

 that each spore of the resulting tetrad receives one. Upon germination 

 the spore produces a gametophyte with one plastid in each cell, and the 

 cycle is complete. 



FIG. 42. The behavior of the plastid in the sporocyte of Anthoceroa. 

 A, sporocyte with single nucleus and plastid. B, plastid divided; nucleus in prophase 

 of mitosis. C, plastids divided to four; two nuclei present. D, three of the four spore 

 cells, each of which has a single nucleus and plastid. (After Davis, 1899.) 



In all of the foregoing examples it is evident that the plastids, as 

 stated by Scherrer for Anthoceros, remain as morphological individuals 

 throughout the whole life cycle, multiplying exclusively by division. A 

 similar claim is made for the plastids of mosses by Sapehin (1915), who 

 has also studied the behavior of the plastids in Selaginella and Isoetes 

 (1911, 1913). In such cases the plastids each possess an individuality com- 

 parable to that of nuclei, from which they differ conspicuously, however, 

 in undergoing no fusion at the time of fertilization. The constancy in 

 number is nevertheless maintained : by the degeneration of the plastids of 

 one gamete in Zygnema; by their failure to divide at one cell-division in 

 Coleochcete; and because of the fact that the male gamete carries no 

 plastid in Anthoceros. It appears to be generally true that while the eggs 

 in all plant groups contain plastids (usually leucoplasts), the latter are 

 present in male gametes in the algae only. Sapehin (1913), however, 

 believes that the blepharoplasts of the higher groups represent plastids. 



It should be said that only in a comparatively few forms has such a 

 regularity in the behavior of the plastid as that outlined above been 

 demonstrated. A number of investigators, working on a great variety of 

 cells, have been forced to conclude that plastids are either formed de novo 

 as well as by division, or are carried through certain stages of the life 



