RELATION TO ENVIRONMENT 



505 



Arrangement of Leaves 



As a general thing, the leaves in most trees are 

 principally confined to the younger branches at the 

 periphery of the head of foliage. To insure perfect 

 exposure to the light we have seen that the arrange- 

 ment of the leaves is such as to prevent shading of 

 "the leaves, either by the alternate arrangement on 

 the stem, or by the lower leaves having longer 

 petioles, which make them project beyond the upper 

 leaves. 



PROTECTION AGAINST COLD 



Plants of the colder regions must provide for 

 protection against extreme cold. Except the Coni- 

 fers, no northern trees are evergreens, and the trees 

 become absolutely dormant during the winter. The 

 fall of the leaves in autumn is no doubt a habit 

 acquired by way of adaptation to winter conditions, 

 and in such trees and shrubs, the growing point of 

 each shoot is securely packed in the winter buds, 

 and tightly covered with thick scale-leaves, which 

 form a very efficient protection to the delicate organs 

 contained within them (Fig. 473). 



Many northern herbaceous plants develop under- 

 ground tubers, bulbs, or rhizomes, from which are 

 sent up in the early spring the rapidly developed 

 shoots and flowers, which often complete their 

 growth within a few weeks and die down to wait 

 until the spring of the following year. This is the 

 case with many of the early spring flowers, like the 

 Bloodroot, Trilliums, Erythroniums, Hepaticas, and 

 many others. These not infrequently grow under 

 deciduous trees, and complete their growth before 

 the leaves of the latter have expanded and thus 

 shut off the light. 



PARASITES AND SAPROPHYTES 



Normal green plants, being able to manufacture 

 organic compounds from C0 2 and water, are inde- 

 pendent of other organisms for their carbonaceous 

 food ; but there are very many plants, especially 

 the Fungi, which, not possessing chlorophyll, are 



If, 



FIG. 473. Popu- 

 lufsbalsanti/cra, 

 showing winter 

 buds protected 

 by thick resin- 

 ous scales. 

 (After BAILEY.) 



