THE AMERICAN BOTANIST 125 



The great advantages conferred by the possession of an 

 herbaceous habit of growth in a region subject to low winter 

 temperatures are obvious, for such plants are able to complete 

 their cycles and to mature seeds in the warm summer months 

 and they can then survive the cold of winter in the form of 

 resistent seeds or by hibernating in the ground. Only the 

 hardier types can maintain permanent aerial stems under these 

 conditions. The more delicate woody families have either 

 been exterminated outright in temperate regions or have sur- 

 vived only by assuming an herbaceous habit and thus flourish- 

 ing in that part of the year which is free from frost. As might 

 be expected if low temperature has indeed been the determining 

 factor in the development of herbs, most of those families 

 which are well able to survive cold as trees or shrubs and which 

 form the bulk of the woody vegetation of the north temperate 

 zone — the willows, birchs, oaks, beeches, walnuts, hickories, 

 wax myrtles, elms, hollies, maples, heaths, buckthorns, lindens, 

 planes, sumachs, cornels, and viburnums — are families which 

 are almost entirely without herbaceous members. Being 

 hardy, they have not been forced to adopt the herbaceous 

 habit. 



As to the details of this change in growth habit, we can- 

 not, of course, be sure, but in those forms which it did not kill 

 outright, the increasing cold probably effected a gradual re- 

 duction in size and an attendant shortening of the time neces- 

 sary to reach maturity until very dwarf forms were produced 

 which were able to develop from seed to seed in a year or two, 

 and which could be killed back to the ground every winter — in 

 short, perennial herbs. The herbaceous vegetation in Arctic 

 and alpine regions today, is still composed almost entirely of 

 such plants. The annual herbs, seem to- have developed from 

 this primitive type under more favorable environment, where 

 a plant growing from seed, and thus without a subterranean 

 food reservoir to give it a rapid start, could become large 

 enough in a single season to reproduce itself. 



