September 1, 1916] 



SCIENCE 



295 



Australia (5,711 species) 



New Zealand (1,026 species) 



Southern South America (1,587 



species). 



South Africa (7,984 species) 



Recent 

 Genera, 

 % Herbs 



Ancient 

 Genera, 

 % Herbs 



Here again, though not to as marked a 

 degree as in insular floras, the more ancient 

 element is predominantly woody and the 

 more recent predominantly herbaceous. 



It is noteworthy that there are many 

 species of plants in the ancient insular 

 floras which are identical or nearly iden- 

 tical with species on widely distant oceanic 

 islands or on ancient continental areas, a 

 fact which strengthens our belief that the 

 vegetation of these regions is a remnant 

 of one which was formerly much more 

 widely spread. 



If the herbaceous element in the vegeta- 

 tion of such isolated regions as we have 

 described is entirely or in great part of 

 recent arrival, we naturally look for its 

 seat of origin to the extensive land areas 

 of the north temperate zone where herbs to- 

 day reach such high development, and where 

 so many new types of animals and plants 

 have had their birthplace. Even here there 

 is evidence that the woody element in the 

 vegetation was at one time much more 

 diversified and prominent than at present, 

 for very many genera and families of trees 

 and shrubs are found here as fossils from 

 the Cretaceous and Tertiary which are ab- 

 sent from the living floras. This is par- 

 ticularly true of Europe, where there are 

 to-day so few species of woody plants. 



These facts — that woody plants are more 

 ancient than herbs as shown by evidence 

 from fossils, from natural relationships and 

 from anatomy; that herbs are now domi- 

 nant and woody plants few in species in 

 regions subject to low winter temperatures, 

 and vice versa; that regions which have 



been isolated from the north temperate land 

 mass possess few herbs in the ancient por- 

 tion of their floras, and that the northern 

 continents supported at no very ancient 

 date a much more varied woody vegetation 

 than at present — all suggest the conclusion 

 that a large portion, at least, of our modern 

 herbaceous vegetation originated in the 

 north temperate zone in response to the 

 progressive refrigeration of climate which 

 we know to have taken place there during 

 the Tertiary. 



The great advantages conferred by the 

 possession of an herbaceous habit of growth 

 in a region subject to low winter tempera- 

 tures are obvious, for such plants are able 

 to complete their cycles and to mature seed 

 in the warm summer months and can then 

 survive the cold of winter in the form of 

 resistant seeds or by hibernating under- 

 ground. Only the hardier types can main- 

 tain permanent aerial stems under these 

 conditions. The more delicate woody 

 families have either been exterminated out- 

 right in temperate regions or have sur- 

 vived only by assuming an herbaceous habit 

 and thus flourishing 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 develop- 

 ment 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, birches, oaks, beeches, 

 walnuts, hickories, wax myrtles, elms, 

 hollies, maples, heaths, buckthorns, lindens, 

 planes, sumachs, cornels, and viburnums — 

 are families which are almost entirely with- 

 out herbaceous members. Being hardy, 

 they have not been forced to adopt the her- 

 baceous 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 



