94 



SCIENCE. 



LN. S. Vol. XVII. No. 420. 



their nutrition. The holophytic forms are 

 noticeably less motile than the others. 

 Thus Euglena, one of the commonest green 

 flagellates, becomes encysted before divi- 

 sion takes place. The resting cell has a 

 firm membrane about it, and closely re- 

 sembles a typical plant cell. The forms 

 without ehromatophores, however, e. g., 

 Scytomonas, may divide longitudinally in 

 the active condition. This difference in 

 motility between the forms with and with- 

 out ehromatophores seems to be the first 

 hint of the differentiation of the charac- 

 teristically motile animals and immobile 

 plants. 



One group of plants (Yolvocaceffi) evi- 

 dently allied to the Flagellata, and some- 

 times even included with them, like ani- 

 mals, show active locomotion during their 

 vegetative existence. Aside from these, 

 and the Peridinete, which may be remotely 

 related to them, locomotion is exhibited 

 only by such reproductive cells as zoo- 

 spores and spermatozoids. The frequent 

 reversion to the motile condition found in 

 the reproductive cells suggests the proba- 

 bility that these have been derived from 

 similar ancestral forms. 



The loss of motility in typical vegetable 

 cells is associated with the formation of 

 a firm membrane, usually of cellulose, 

 about the cell. This precludes all move- 

 ment of the cell, except in those cases 

 where openings are present, through which 

 extensions of the protoplasm, usually in 

 the form of cilia, protrude. 



The power of free locomotion was prob- 

 ably a character of the primitive vegetable 

 cell, but with the development of the holo- 

 phytic habit, this power has been lost by 

 the vegetative cell of most plants. The 

 loss of locomotion in plants may probably 

 be connected with the development of the 

 power to assimilate carbon dioxide, the 

 main source of food. As the CO2 in the 

 air, or dissolved in water, is constantly 



being received, it is not necessary for the 

 plant to move from one place to another 

 in search of food, and we find plants be- 

 coming more and more stable. Where ani- 

 mals are so placed that their food supply 

 is abundantly received, they may assume 

 an immobile plant-like habit. This is 

 especially marked in many marine animals, 

 such as corals, hydroids, sponges, ascidians 

 and such molluscs as oysters. The old 

 name 'zoophyte' applied to corals and sim- 

 ilar animals was not in all respects a mis- 

 nomer. These rooted marine animals ex- 

 hibit another resemblance to plants in the 

 development of free swimming larvas, an- 

 alogous to the active zoospores produced 

 by so many algse. In both instances it 

 is safe to assume that the motile stage is 

 older than the fixed condition. 



Lack of time forbids our consideration 

 in detail of the very important, but by no 

 means clearly understood, problems deal- 

 ing with the evolution of sex in the vege- 

 table kingdom. Thus the reason why the 

 development of distinct sexual cells has 

 taken place in an almost identical manner 

 in several widely separated groups of 

 plants is hard to explain. The sexual 

 cells, or gametes, have beyond question 

 been derived from non-sexual ones. Thus 

 in several groups of algte; e. g., Volvoca- 

 ceffi, ConfervoidcEe and Phseophyce^, there 

 still exists an almost perfect series of forms 

 leading from the non-sexual zoospores to 

 perfectly differentiated male and female 

 gametes. The formation of sexual or non- 

 sexual reproductive elements is, in many 

 cases at least, largely dependent upon the 

 conditions under which the plants are 

 grown. This has been very clearly shown 

 by the remarkable series of investigations 

 made by Professor Klebs upon various 

 thallophytes. For a discussion of the 

 meaning of sex, the reader may refer to 

 the recent papers on the subject by Stras- 

 burger and Beveri. 



