igoS.] OF THE ANCESTORS OF THE PLANT KINGDOM. 93 



to the formation of a tissue or filament. The other two conditions 

 are reduced or transient. 



Having established these three principal " conditions," Chodat 

 proceeded to establish the phylogeny of the green algae along these 

 lines and succeeded in clearing up considerable obscurity which had 

 previously existed. However, the starting point selected by Chodat 

 has been open to some criticism and it remained for Blackman to 

 suggest the most satisfactory explanation of the origin of this 

 group. He, while following in a general way the theory of Chodat, 

 took the position that the three " tendencies " had their origin not in 

 the non-moile Pahnella form, but in the motile Chlamydomonas type. 



I have had the genus Chlamydomonas under investigation for 

 several years, observing its various species for the most part in pure 

 cultures grown upon both solid and liquid media. The vegetative 

 cells of Chlamydomonas are variable in both size and shape ; in gen- 

 eral, however, they are from 20-35 /a i" length and 10-20 ix in 

 breadth, being elliptic or pyriform in outline. One end of the cell 

 is usually produced into a colorless beak, from which two cilia 

 always protrude. The chloroplast is quite variable in form and 

 with one exception is provided with a single pyrenoid. Non-sexual 

 reproduction is by means of zoospores, which are formed by the 

 division of the contents of the mother cell, after it has come to rest. 

 Sexual reproduction is usually by the conjugation of naked motile 

 gametes of similar size and in no way distinguishable from each 

 other. It is interesting to note, however, that in addition to this 

 method there may also be the conjugation of unequal motile gametes 

 and in one species — to be referred to later — there takes place the 

 conjugation of dissimilar gametes, one of which, the larger, comes 

 to rest before conjugation. We thus have within the limits of this 

 well defined and natural gerais, not only the most primitive form 

 of gamogenesis, but through anisogamous conjugation a gradual 

 approach to true oogamy — the highest type of sexual reproduction 

 developed among the algas. 



In abandoning the starting point of Chodat's theory of the devel- 

 opment of the green algse, it is not necessary to replace his idea 

 relative to the three predominating tendencies manifest in the lower 

 members of this group. While different names are attached to these 



