MODIFICATIONS OF FORM 195 



has been acquired in species that have spread to regions with marked seasons. 

 On the other hand, in hotter climates than our own, a dry and hot season may 

 also be tided over by many plants by a fall of their leaves, and a new suit 

 of leaves is usually formed after the commencement of the rains. The 

 physiological advantage is similar to that of our autumn leaf-fall at home. 

 Protection against drought is gained. But the drought thus met in the tropics 

 is a real lack of water in hot weather. In very hot dry seasons our own trees 

 sometimes drop their leaves in the same way. Thus by a simple modification 

 plants can limit their transpiring area temporarily. However prominent 

 the fall of the leaf in autumn may appear to be, it is not a fundamental 

 feature, but only a special adaptation to season. 



The annual habit may similarly be regarded as an accommodation 

 to seasonal change. The seed is more resistant to extremes of 

 temperature and drought than the growing plant. If then the 

 vegetative development, from germination to flowering and fruiting, 

 can all be completed within one growing season, an adverse period 

 can be safely passed as seed, and the species will survive. Practically 

 this has proved more effective in temperate than in tropical climates, 

 as is shown by the prevalence of annuals in the temperate Flora. On 

 the other hand, annuals are few in forest areas, which are less favour- 

 able to their growth than open ground. It is worthy of note that the 

 Arctic and Alpine Floras consist almost entirely of perennials. This 

 is easily understood, since the vegetative season is there too short for 

 the completion both of vegetation and propagation. The fact is 

 illustrated by the Alpine Flora of the Scottish Hills, which is distinc- 

 tively Arctic in its character. 



Perennation and Storage. 



Perennation, that is the maintenance of the individual from year 

 to year, presents no difficulties where the seasons are equable, as in 

 many tropical areas: here perennials, growing steadily on from 

 year to year, form a leading element in the Flora. But in temperate 

 regions, with their strongly marked seasons, various adaptations of 

 the vegetative shoot besides that of leaf-fall may be seen, especially 

 in herbaceous plants, for tiding over the winter. They are associated 

 with the storage of material, which is thus carried over from one season 

 to the next (Chapter VIII.). The simplest case is that of biennial 

 plants, such as the Evening Primrose and Foxglove ; or, among culti- 

 vated plants, the Turnip, Carrot, Beet-root, or Onion. These in their 

 first year store their surplus nutriment in the vegetative organs at the 

 base of the plant, and use it up in flowering and fruiting in the following 



