70 INTRODUCTION TO CYTOLOGY 



independent of that of the nucleus and cell, although they may assume a 

 characteristic polarized arrangement about the mitotic figure (Fig. 25,/). 



It was believed by the early observers, notably Schimper (1883) and 

 Meyer (1883), that plastids never originate de novo but always arise from 

 preexisting plastids by division. This view has the recent support of 

 Bowen (1929a). Fully differentiated plastids, such as chloroplasts, can 

 readily be seen multiplying in this manner in growing tissues with a 

 frequency sufficient to account for the large number of plastids present 

 in mature plant parts. However, since the individual plant arises from a 

 zygote or a spore in which the plastids are usually in a relatively undiffer- 

 entiated state, the problem of the individuality of the plastid is mainly 

 one of determining whether these undifferentiated "plastid primordia," 

 later developing into chloroplasts and other types, are continuous through 

 the critical stages of the life cycle, multiplying only by division, or arise 

 de novo as new differentiations of the cytoplasm. 



Light on this question has been sought in a number of investigations 

 on bryophytes."^ In Anthoceros each cell of the gametophyte contains a 

 single plastid which divides with the nucleus at each cell-division. The 

 egg likewise contains a plastid, but apparently the spermatozoid has 

 none; the zygote and sporophyte cells which it later forms are, therefore, 

 like the cells of the gametophyte, characterized 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. The sporo- 

 cyte plastid divides twice as the sporocyte begins to divide into a quartet 

 of spores so that each spore receives one plastid. Upon germination the 

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

 is complete (Davis, Scherrer). According to Scherrer, the plastids 

 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. 



More recent studies have added much to our knowledge of plastid 

 behavior in the reproductive phases. In Polytrichum commune, Weier 

 has followed the plastids through both sporogenesis and spermatogenesis. 

 The cells of the young sporophyte contain several small plastids which 

 multiply by division, and in each sporocyte there are usually two larger 

 ones. In fixed preparations these show a deeply staining peripheral 

 portion ("plastonema") and a fighter internal region ("plastosome") 

 (Fig. 34). They pass through a temporary phase in which they appear 

 loose and granular; they then divide to form four which are included in 

 the four spores as the sporocyte divides. In the spore the plastid soon 

 assumes a structure characteristic of the chloroplast. In the antherid- 



21 Davis (1899), Scherrer (1914), Sapehin (1915; see also 1911 and 1913, on 

 pteridophytes), Senjaninova (19276), Motte (1928), Weier (1930a, 1931a6), Chalaud 

 (1929-1930). 



