296 



SCIENCE 



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



the increasing cold probably effected a 

 gradual reduction in size and an attendant 

 shortening of the time necessary to reach 

 maturity, until very dwarf forms were pro- 

 duced 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 to-day is still composed almost en- 

 tirely of such plants. The annual herb 

 seems to have developed from this primi- 

 tive type under more favorable environ- 

 ments, where a plant growing from seed, 

 and thus without a subterranean food res- 

 ervoir to give it a rapid start, could become 

 large enough in a single season to reproduce 

 itself. 



The northern vegetation thus developed 

 proved extremely hardy and aggressive, 

 and was able not only to overspread the 

 great continental area of the north tem- 

 perate zone but to invade as well the tropics 

 and even the Antipodes. The presence of 

 a large number of typically northern 

 genera of plants in Australasia, southern 

 South America and South Africa, often 

 separated from their related forms by the 

 whole width of the tropics, has long been 

 recognized as one of the most fascinating 

 problems of plant distribution. It is im- 

 portant to note that this invasion of north- 

 ern plants (nearly 200 genera are known) 

 which has been so successful in penetrating 

 far southern regions and which displays so 

 well the ' ' wonderful aggressive and coloniz- 

 ing power of the Scandinavian flora" to 

 which "Wallace and others have called at- 

 tention, has in reality been an invasion of 

 herbs, for almost none of the northern trees 

 and shrubs have participated in it. 



Herbaceous plants have also been de- 

 veloped in the south temperate zone appar- 

 ently in response to the refrigeration of 

 climate there in the late Tertiary. Ant- 



arctic herbs were doubtless among the very 

 last plants to leave the polar continent as 

 the glaciers advanced. They are still 

 almost all alpine or cold-loving perennials 

 and have as yet failed to give rise to the 

 aggressive lowland annual type. 



Kefrigeration of climate was doubtless 

 not the only factor in the development of 

 an herbaceous vegetation. A large body 

 of such plants seem to have originated in 

 arid regions, where they spring up rapidly 

 and produce seed during a rainy season, 

 thus bearing precisely the same relation to 

 extremes of moisture that arctic or alpine 

 herbs do to extremes of temperature. The 

 assumption of a rapidly climbing habit, 

 especially in the tropics, has also resulted 

 in the development of an herbaceous type 

 of stem in such families as the melons, milk- 

 weeds and passion-flowers. 



But whatever the cause of their origin, 

 herbs have proved themselves an exceed- 

 ingly versatile and aggressive type of vege- 

 tation under almost all climatic conditions. 

 The reasons for this dominance of the herb 

 are not far to seek. It is able not only to 

 thrive in cold and arid regions but, from 

 the brevity of its life-cycle, can take advan- 

 tage of temporarily favorable conditions of 

 any sort. Its evident and great superiority 

 over woody plants in rapidity of dispersal 

 and ability to invade new areas quickly is 

 due in large measure to the fact that its 

 interval from seed to seed, instead of being 

 many years, is only a few months. Every 

 seed may itself become a center of dispersal 

 in a season's time. The amount of seed 

 produced, too, in proportion to the bulk of 

 plant body which has to be developed is far 

 greater among herbs than among woody 

 forms. Owing to the rapid multiplication of 

 their generations herbs are capable of more 

 rapid evolutionary change than are trees 

 or shrubs and hence are able to adjust 

 themselves more rapidly to new conditions. 



