January 16, 1903.] 



SCIENCE. 



93 



hand statement that we see simply clouds 

 floating in the atmosphere of the planet. 

 G. W. Hough. 



THE ORIGIN OF TERRESTRIAL PLANTS.* 

 I SHOULD like to invite your, attention 

 for a little while to some of the factors 

 that apparently have been operative in 

 determining the changes which plant 

 structures have undergone in the course 

 of the development of the vegetable king- 

 dom. While some of these are perfectly 

 obvious, others are by no means so evident, 

 and, as might be expected, there is not per- 

 fect agreement among botanists as to the 

 relative importance of some of these fac- 

 tors, nor indeed of their efficiency at all. 

 I shall not attempt here to go into any 

 extended discussion of the remarkable re- 

 sults obtained by Professor De Vries in 

 his recent studies upon variation in plants. 

 These are too important, however, to be 

 dismissed without some mention. The 

 conclusion reached by Professor De Vries 

 is that, in addition to the variation within 

 the limits of species, there may be sudden 

 variations, or 'mutations,' which, so to 

 speak, overstep the limits of the species, 

 and thus inaugurate new species. While 

 the results obtained, especially in the ease 

 of CEnothera Lamarckiana, are certainly 

 most striking, more data are necessary be- 

 fore we can accept without reserve the 

 conclusions reached. It is certain that 

 marked changes— 'sports,' as the garden- 

 ers term them — often appear without any 

 explainable cause, and it is equally diffi- 

 cult to understand, what for want of a 

 better term, we can only term 'tendencies' 

 to develop in special directions. Thus 

 the specialization of the sexual reproduc- 

 tive cells, which has evidently taken place 



* Address of the chairman of Section G, Botany, 

 and vice-president of the American Association for 

 the Advancement of Science. Read at the Wash- 

 ington meeting, December 29, 1902. 



quite independently in several unrelated 

 lines ; the development of heterospory, and 

 probably of the seed-habit in different 

 groups independently, are hard to explain 

 without assuming an innate tendency to 

 vary in a determined direction. 



It is not, however, with these exceed- 

 ingly difficult and often obscure problem's 

 that we shall concern ourselves here, but 

 rather with those changes in plant struc- 

 tures which are referable to more or less 

 evident response to known conditions. 



Speaking in broad terms, I think we 

 can reduce the determining factors to 

 three categories, leaving aside the inherent 

 tendencies to variation. These three sets 

 of factors are: (1) those relating to the 

 food supply, (2) the relation to water 

 and (3) those concerned with reproduc- 

 tion. 



It is hardly necessary to say that there 

 is no fundamental distinction between 

 plants and animals. At the bottom of 

 the scale of organic life are many forms, 

 especially those belonging to the group of 

 Flagellata, which are intermediate between 

 the strictly animal and vegetable organ- 

 isms. 



We may safely assume that the primi- 

 tive organisms were motile, perhaps re- 

 sembling some of the existing flagellates. 

 Of the latter some are destitute of pig- 

 ment and approach the lower Protozoa; 

 others are provided with chromatophores 

 containing chlorophyll and resemble the 

 lower plants. It is highly probable that 

 the forms with chromatophores are able 

 to assimilate carbon dioxide, as the typical 

 plants do, and may be denominated 'holo- 

 phytie. ' The forms without chlorophyll 

 are probably, like animals, dependent upon 

 organic food for their existence. 



If we compare the holophytic flagellates 

 Avith those forms which have no chloro- 

 phyll, a significant difference may be 

 noted, which is evidently associated with 



