THE LEAF. - 
tissue,” as it is called, has a double function : it acts as a water 
reservoir, and it protects the chlorophyll of the palisade paren- 
-chyma from too great exposure to light. It is interesting to 
notice that the position of the chlorophyll corpuscles in the 
cells varies with the intensity of light. When expesed to 
diffuse daylight, the corpuscles are found on the upper and 
lower walls of the palisade cells; but if diffuse daylight 1s 
replaced by strong sunlight, the corpuscles are removed to the 
lateral walls and towards the inner end of the cell. 
The numerous contrivances against too rapid transpiration 
may now be summed up: 
1. The cuticle, which is thin in a Mesophyte, is always 
thick in a Xerophyte. 
Holly is a good instance AS ae 
of this. The thickness ~~ = ea 
of the cuticle is due to Ie Nt | C5 
the development of a OOO! 
substance called cutin, SQ CS 
Owing to whichthecell.  % Deo 
SS 

wall becomes less and Ye YC A ie 
less permeable to water. casi 
A very thick cuticle is Fic. 43.—Lear or Houty, SECTION OF 
Upper SIDE OF BLADE. (High power.) 
- al i 2 . 
ways found on trees e, epidermis ; ¢, cuticle. 
Where the leaves re- 
main several years on the branches, such as Holly, Oleander, 
Mistletoe, Cactus. 
2. Some plants, especially those which grow on steppes and 
prairies, develop a waxy covering in the cell-walls of the epi- 
dermis. This may be seen in young Cherry, Apricot, and Peach 
trees, where the young leaves are thus protected ; but as soon as 
they burst from the bud, and the cuticle is fully developed, it 
disappears, so that transpiration may not be hindered. 
3. Many plants have a hairy covering on the upper side of 
the leaf. This is especially the case in alpine plants, which 
are exposed to dry air only for a few hours each day, and to 
frost and cold at night. They require to be able to lessen 
transpiration, owing to the difficulty of getting water in 
the soil in which they grow. Thus, the Edelweiss has a 
