THEIR STRUCTURE AND FORMS. 87 



season earlier than the last ; and the branches are continually 

 clothed with the foliage of from two to five, or even ten or more 

 successive years. On the other hand, it is seldom that all the 

 leaves of an herb endure through the whole growing season, the 

 earlier foliage near the base of the stem perishing while fresh 

 leaves are still appearing above. In our deciduous trees and 

 shrubs, however, the leaves of the season are mostly developed 

 within a short period, and they all perish in autumn nearly 

 simultaneously. 



163. Leaves soon complete their growth, and have no power 

 of further increase. Being organs for transpiration, a very large 

 part of the water imbibed by the roots is given out by the foliage, 

 leaving dissolved earthy matters behind. Assimilation can take 

 place only in fresh and vitalty active tissue. It is incident to all 

 this that leaves should be of only transient duration, at least in 

 their active condition. 



164. Defoliation. The leaves of most Dicotyledons and some 

 Monocotyledons separate from the stem and fall by means of an 

 articulation at the junction with the stem, which begins to form 

 early in the season and is completed at the close. There is a kin<l 

 of disintegration of a transverse layer of cells, which cuts off thn 

 petiole by a regular line, and leaves a clean scar, such as is seen 

 in Fig. 81, 85, 91. Some leaves, notably those of Palms 

 Yucca, and other endogens, die and wither on the stem, or wear 

 away without falling. 



165. In temperate climates, defoliation mostly takes place at 

 the approach of winter. In warmer climates having only winter 

 rain, this occurs in the hot and dry season. 



166. Normal Direction or Position. The leaf-blade is expanded 

 horizontally, that is, has an upper and an under surface. When 

 erect, the upper surface faces the axis which bears it. To this, 

 there are many seeming but no real exceptions ; that is', none 

 which are not explicable as deviations or changes from the normal 

 condition. (213-217.) 



2. THEIR STRUCTURE AND FORMS AS ORGANS OF ASSIMILATION 

 OR VEGETATION, i. e. AS FOLIAGE. 



167. The Internal Structure or Anatomy of the leaf needs here 

 to be examined so far as respects its obvious parts and their 

 general composition. The leaf, like the stem, is composed of 

 two elements (131), the cellular and the woody. The cellular 

 portion is the green pulp or parenclrpna, and in this the work 

 of assimilation is carried on. The woody is the fibrous frame- 



