39 2 RELATION TO ENVIRONMENT. 



t 



II. Protective Modification of Leaves. 



762. General directions in which these modifications have 

 taken place. The usual type of foliage leaf selected is that of 

 deciduous trees or shrubs or of our common herbs. Such a 

 leaf is usually greatly expanded and thin in order to present as 

 great a surface as possible in comparison with its mass, since 

 the kind of work which the leaf has to do can be more effectu- 

 ally carried on when it possesses this form. This form of leaf 

 is best adapted for work in regions where there is a medium 

 amount of moisture such as exists in the temperate zones. But 

 since there are very great variations in the climatic and soil 

 conditions of these regions, and even greater changes in desert 

 and arctic regions, the type of leaf described is unsuited for 

 all. Its own life would be endangered, and it would also en- 

 danger the life of the plant. Modifications have therefore taken 

 place to meet these conditions, or at least those plants whose 

 leaves have become modified in those directions which are 

 suited to the surrounding conditions have been able to persist. 

 Excessive cold or heat, drought, winds, intense light, rain, etc., 

 are some of the conditions which endanger leaves. The pro- 

 tective modifications of leaves may be grouped under four gen- 

 eral heads: (i) Structural adaptations; (2) Protective cover- 

 ing; (3) Reduction of surface; (4) Elimination of the leaf through 

 the complete assumption of the leaf function by the stem. 



763. (i) Structural adaptations. The general structure of 

 the leaf presents certain features which are protective. The pali- 

 sade layer of cells found usually beneath the upper epidermis 

 forms a compact layer of long cells which not only acts as a 

 light screen cutting off a certain amount of the light, since too 

 intense light would be harmful ; it also aids in lessening the loss 

 of water from the upper surface, where radiation is greater. 

 The stomata are usually on the under side of aerial leaves, and 

 the mechanism which closes them when the leaf is losing too 

 much water is protective. As a protection against intense light 

 the number of palisade layers is sometimes increased or the 



