i62 BOTANY OF THE LIVING PLANT 



Biology of Season and of Duration. 



If we attempt to sketch a general, that is a non-speciahsed type of 

 Flowering Plant, it would have a cylindrical upright stem, bearing 

 leaves with petiole and lamina radiating out on all sides of it, and with 

 axillary branches repeating the characters of the main shoot. Its 

 root-system would consist of a tap-root and lateral roots of successive 

 orders, all fibrous. A young Sycamore or Apple-tree would answer 

 this general description. Further, it seems probable that the perennial 

 state was prevalent, or even constant among early Vascular Plants. 

 For it is seen almost exclusively in living Pteridophytes and Gymno- 

 sperms, and it is characteristic of the early fossils. So that in this 

 respect, as also in their general form, an Apple or a Sycamore may be 

 held as representing a type of vegetative construction usual for early 

 Flowering Plants. 



In one marked feature, however, the Sycamore and the Apple are cer- 

 tainly adaptive. Both are deciduous^ that is they drop their leaves in 

 autumn, as do most of our British trees and shrubs. Leaf -f all \s clearly 

 related to season. It brings the biological advantage of reducing the 

 transpiring area at the time of low temperature, when the activity of the 

 roots in the cold soil declines. It is in fact a provision against what 

 may be called physiological droughty for the roots in the cold soil in 

 winter are unable to make up for any great loss of water by transpira- 

 tion. But many familiar plants retain their leaves during the winter, 

 as " evergreens." They are mostly shrubs with leathery leaves, and 

 many of them have been introduced from southern lands, such as the 

 Rhododendron and Cherry-Laurel, from the Levant ; but Holly, and 

 Yew, and Ivy are native evergreens. 



The evergreen state is more common in plants of lands where the seasons 

 are equable, and it is probably a primitive state, while the deciduous habit 

 has been acquired in species that have ^-^read 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 b}- 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 explanation is similar to that of our autumn leaf-fall at home. 

 It is a protection against drought. 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 



li 



