CHAPTER IX 



THE LEAF IN RELATION TO ITS EXVIROXMENT 



A change in the surroundings of any plant can so react upon it as to cause it to change. By the 

 attempt, conscious or unconscious, to adjust itself to the new conditions, a true physiological change 

 is gradually wrought within the organism. PROFESSOR DRUMMOND. 



A LTHOUG-H the previous chapter was devoted chiefly to the 

 -^V- consideration of the forms of leaves, we must now briefly 

 resume the subject in order to refer to a few forms not hitherto 

 noticed curious and exceptional forms, of which, in most cases, our 

 British plants afford no examples. This will pave the way to the subject 

 more especially before us namely, the adaptation of foliage leaves to 

 their environment. 



The subject of environment, in so far as the sustaining of vegetable 

 life and vigour is concerned, has been already dealt with in preceding 

 chapters, where we have seen that, while in the plant itself resides the 

 principle of Life, in the environment are found the conditions of Life ; 

 and that without the fulfilment of those conditions in other words, 

 without the regular supply of heat, air, water, inorganic substances, 



and so forth, to the living 

 tissues the plant would 



f't^Jr^"^ ***- -^ languish and die. This part 



tiJf&f^ ' of the ground the most 



IjUjrs S?^ important part without 



jJPJS^ OA *, doubt we do not propose 



/v^C" x ^ to retrace. What will be 



$&' V*C^ before us in the pages im- 



f ' ^$ mediately succeeding is the 



effect of environment in 

 modifying the structure 

 rather than in sustaining 

 the life of the plant the 

 effect, indeed, which is evi- 

 dent in what is called Vari- 

 ation. This ma}' appear to 

 FIG. 340. LEAF OF A Laportea. be anticipating, but many 



With a cup-like enlargement of the extremity of the mid-rib. of the morphological facts 



278 



L ) 



