242 



LEAVES 



these phenomena. Similarly, the edgewise position of 

 leaves with reference to light is protective with reference 

 to excessive light and with reference to excessive transpira- 

 tion as well. 



Almost every part of a plant has its protective aspect. 

 In leaves we have considered the protective nature of the 

 epidermis and of the palisade tissue, and we may say of 

 the veins that they protect the otherwise soft blades of 

 the leaf from being torn by the winds. We have noted 

 that the expanded nature of the leaf, and the necessary 

 exposure of its delicate inner cells to the air, make it more 

 subject to dangers than the root or the stem. Lack of 

 water, lack of Kght, and lack of warmth appear to be the 

 principal dangers to which the leaf is exposed, and of these 

 the first named is by far the greatest. Many different 

 means of protection from these dangers are found. Having 

 in mind the kind of protection a leaf needs, as well as the 

 kind of work which it does, we are able usually to perceive 

 the advantages' or the disadvantages of the various kinds 

 of structures which we meet. 



A. The Shedding of Leaves and their Autumnal Colors. — 

 The regular fall of leaves from the plant is a very impor-* 

 tant means of protection. It is, of course, not a means 

 of protection to the. leaves themselves, but as a means of 

 protection to the plant as a whole this habit is of very 

 great advantage. 



The most striking difference between the plants with 

 which we are familiar and the plants of the tropics is that 

 with us plants lose their leaves in the winter, while tropical 

 plants generally bear leaves the year round. Plants which 

 have the habit of losing all their leaves at about the same 



