20 BRITISH FLOWERING PLANTS chap. 



In considering the importance of these provisions 

 we have not only to bear in mind the desirability of the 

 seeds being scattered beyond the shadow of the parent 

 plants, and the soil which has been more or less exhausted 

 by them, but must also remember the vicissitudes of 

 climate through which our islands have passed. Before 

 the glacial period our flora seems to have been, on the 

 whole, very much what it is now. During the glacial 

 period our plants were driven south, a few of the Arctic 

 and Alpine species only remaining in the extreme south 

 and west. During this period there seems to have been 

 certainly one, and possibly several comparatively mild 

 periods, so that the whole, or nearly the whole, of our 

 plants have been compelled at least twice, probably 

 several times, to migrate long distances.^ Perhaps for 

 this reason our flora is highly specialised for dispersal, 

 or rather the species best organised in this respect are 

 those which have been able to re-establish themselves 

 here. 



The methods of packing the embryo in the seed-coat, 

 the germination of the seed, and the characters of the 

 seedling afford interesting subjects for study. Some 

 account of these will be found in my books on Seedlings. 



Leaves 



The leaf originates as a slight protuberance of the 

 bud. Fig. 6 represents a longitudinal section through 

 the growing. point of a winter bud of Abies pectinata 

 (Silver Fir), and shows the commencement of two 

 leaves [b, b). It also illustrates the cellular structure 

 of young tissue. 



The nourishment absorbed by the roots from the 

 soil passes as crude sap into the leaves and is there 

 elaborated. Carbonic acid gas is absorbed from the air 

 by means of the stomata, — minute slit-like apertures 

 between two epidermal cells which guard the entrance 

 (guard-cells), and, by contraction, close it at night and 

 in dull weather. The stomata allow communication 



' Roid, Origin of the British Flora. 



