No. 463.] STUDIES ON PLANT CELL. VI. 45 1 



possibilities in the thallophytes which must remain as specula- 

 tions until investigations have advanced much farther in this 

 difficult field of cell study. The basis of any theories at present 

 must be phylogenetic, a principle that has not been followed in 

 some of the work upon the thallophytes. 



Gametogenesis in plants is full of interest because of the 

 sharp differences from the processes of spermatogenesis and 

 oogenesis in animals. In animals the period of gametogenesis 

 is one of unusual activity. After the germ cells are differenti- 

 ated there follows a period of cell growth, with the peculiar 

 activity termed synapsis, during which the number of chromo- 

 somes is reduced to one half the number characteristic of the 

 species. The germ cells emerge from the growth periods as 

 primary spermatocytes or oocytes which give rise respectively 

 by two successive mitoses to four spermatids or to an egg with 

 its accompanying polar bodies. The gametes have one half the 

 number of chromosomes characteristic of the species, so that 

 the period of gametogenesis is one of chromosome reduction. 

 The character of this process of reduction will be considered 

 when we take up the analogous phenomena in plants after the 

 discussion of sporogenesis. Gametogenesis in plants is in strik- 

 ing contrast to that in animals. In all higher groups (those 

 above the thallophytes) we know that the gametes have the 

 same number of chromosomes as the vegetative cells of the 

 parent plant (gametophyte). There is no reduction of the chro- 

 mosomes at the time of gametogenesis, that phenomenon taking 

 place at the end of the sporophyte generation with sporogenesis. 

 Also, there are no peculiarities of the mitoses immediately 

 preceding gametogenesis excepting such as concern the devel- 

 opment of cilia-bearing organs (blepharoplasts) or slight pecul- 

 iarities in the form or size of the spindles, for such nuclear 

 figures are frequently different in these particulars from the 

 mitoses in vegetative cells of the gametophyte. The differences 

 concern chiefly the structure of the sperm, and have been de- 

 scribed in our account of that structure (Amer. Nat., vol. 38, 

 July and August, p. 576, 1904). 



To Strasburger above all others should be given the credit 

 of making clear these important characteristics of gametogene- 



