24 A TEXT-BOOK OF BOTANY 



with definitely spaced lines, preferably five millimeters 

 apart.* As the leaf continues to grow, the most active 

 growing region will be indicated by the lines that draw 

 farthest apart. 



17. Protection. Such an important organ as the leaf, 

 with its delicate active cells necessarily in communication 

 with the air, is exposed to numerous dangers. Conspicu- 

 ous among these dangers are drought, intense light, and 

 cold. Many ways of meeting these dangers have been 

 developed by plants, but the subject is too large and com- 

 plex to be presented with any completeness. The best 

 that can be done is to select a few striking illustrations of 

 protection that seem to be definite. Perhaps the most 

 common danger to most plants is an excessive loss of water, 

 and when a drought prevails the problem of checking trans- 

 piration is a most serious one. As the leaves are the 



prominent transpiring organs, 

 the chief methods of protec- 

 tion concern them. 



A B 



FIG. 18. Sections through leaves of the same plant, showing the effect of exposure 

 to light upon the structure of the mesophyll : A , leaf exposed to intense sun- 

 light ; B, leaf grown in the shade. After STAHL,. 



(1) The epidermis may be regarded as an ever-present 

 check against transpiration (Fig. 12), for without it the 



* Such scales on stem and root are seen in Figs. 57 and 75. 



