292 



SCIENCE 



[N. S. Vol. XLIV. No. 1131 



But however valueless an inquiry which 

 concerns these more conspicuous distinc- 

 tions may be in tracing outlines of descent 

 and determining the evolution of flora, it 

 does provide us with important informa- 

 tion as to the origin and development of 

 plant forms, the evolution of vegetation. 

 In many ways this knowledge is of more 

 importance than the construction of family 

 trees alone, for it is more often the growth 

 habit of plants rather than their systematic 

 position which is correlated with the cli- 

 matic, geological and zoological factors in 

 their environment. Indeed, to man himself 

 the distinction between an herb and a tree 

 is frequently of greater economic signif- 

 icance than that between two families of 

 plants. 



Investigations on this problem of the 

 evolutionary history of growth forms 

 among the higher plants has produced evi- 

 dence from various sources that in com- 

 paratively recent geological time there has 

 been a radical change in the character of 

 much of the earth's vegetation, perhaps the 

 most important one since the appearance 

 of the angiosperms; a change produced by 

 the origin and wide dispersal of those lowly 

 but numerically abundant, quickly matur- 

 ing and rapidly spreading plants, the 

 herbs, in a vegetation which seems to have 

 been previously composed almost entirely 

 of trees and shrubs. Light has also been 

 thrown on the factors which were respon- 

 sible for the development of this new plant 

 type and on the far-reaching changes which 

 its introduction has caused in the history 

 of plants, animals and man. 



In any such problem of evolution as 

 this we naturally turn first to a study of 

 the fossil record. Of course the very 

 earliest of land plants, if our present 

 theories are correct, were delicate semi- 

 aquatic species, probably resembling our 

 modern liverworts, plants which from their 



essentially herbaceous structure failed en- 

 tirely of preservation. As to the develop- 

 ment of these lowly forms into the vigorous 

 and land-loving vascular plants which are 

 now so completely dominant we know 

 almost nothing, either from the geological 

 record or from the occurrence of inter- 

 mediate types. The luxuriant vegetation 

 of the latter part of the Paleozoic, which 

 gave us our first fossil plants, was com- 

 posed of various ancient types of ferns, 

 lycopods, horse-tails, cycad-ferns and gym- 

 nospermous seed plants. It is significant 

 that although nearly all of these were 

 either trees or stout woody forms, their 

 representatives which have been able to 

 survive to the present time have with few 

 exceptions been reduced to such an her- 

 baceous stature as characterizes our ground 

 pines, horse-tails, quillworts and ferns 

 to-day. Among the angiosperms, which oc- 

 cur as fossils only since the lower Cre- 

 taceous, a similar change seems to have oc- 

 curred for nearly every one of the early 

 fossil members of this great group were 

 apparently woody plants. Of course it 

 must be borne in mind, as in all such cases, 

 that the geological record may not present 

 us with a fair sample of an ancient flora; 

 for the leaves of woody species would in 

 general be much more favorable for pres- 

 ervation than the more delicate ones of 

 herbaceous plants. As far as it goes, how- 

 ever, the geological evidence tends to indi- 

 cate that among vascular plants, at least 

 during the period covered by the earlier 

 fossil record, woody forms were the domi- 

 nant type of vegetation. 



The labors of the botanical taxonomists 

 have also provided us with a valuable clue 

 as to the history of growth habits, particu- 

 larly among the angiosperms. This now 

 dominant race is generally agreed to have 

 descended either from cycad-like types or 

 from forms related to the conifers. Both 



