LEAF AND STEM STRUCTURE OF THE OLIVE. 49 



LEAF AND STEM STRUCTURE OF THE OLIVE. 



The structure of the olive leaf is that of a xerophyte; in other 

 words, it shows in a high degree pecuharities of structure that char- 

 acterize most woody plants that grow in situations where both air 

 and soil normally contain a relatively small amount of moisture. 

 On the upper surface of the leaf the cuticle and outer walls of the 

 epidermis cells are greatly thickened, stomata are absent, and shield- 

 shaped hairs are scattered over the surface. On the lower face the 

 outer walls of the epidermis cells are very thick (though less so than 

 on the upper surface), the stomata are placed at the bottom of nar- 

 row pits, and shield-shaped hairs form a dense continuous covering. 

 The interior, chlorophyll-bearing tissue (chlorenchyma) consists of 

 three or four very compact layers of palisade cells (i. e., narrow cells, 

 elongated at right angles to the epidermis) beneath the upper epi- 

 dermis, and between the palisades and the lower epidermis many 

 layers of so-called pneumatic tissue, the cells of which are very irreg- 

 ular in shape, not much longer than wide, and inclose numerous air 

 spaces. Prosenchymatic cells with very thick walls (the stereome), 

 either singly or in groups, are scattered through the mesophyll and 

 occur here and there directly beneath the epidermis, as well as in 

 several continuous layers adjoining the midrib. Between the mid- 

 rib and the sheath of stereome there is no chlorenchyma, but extend- 

 ing to the epidermis on both sides are several layers of collenchyma, 

 of which the cells contain no chlorophyll and have their walls greatly 

 thickened, especially at the angles. 



Of the foregoing characters, those which may be pointed out as 

 especially xerophytic are: Thickness of the cuticle and outer cell 

 walls of the epidermis, absence of stomata on the upper surface and 

 their situation in pits on the lower face, and the dense covering of 

 flat, shield-shaped hairs on the lower face. These characters are 

 supposed to be especially useful to plants that inhabit dry climates 

 or that grow in soils from which their roots obtain moisture with 

 difficulty, by protecting the leaves from excessive loss of water 

 through transpiration. The development of the chlorenchyma be- 

 neath the upper face of the leaf into several layers of compact pali- 

 sade tissue is also characteristic of many xerophytes. 



In leaves of the olive developed in the shade or in a moist atmos- 

 phere, the cell walls of the epidermis are much thinner, the stomata are 

 level with the surface instead of being situated in pits, and the midrib 

 is embedded in chlorenchyma, with a much smaller development of 

 collenchyma. 



Leaves and young twigs of olive trees were collected in abandoned 

 orchards at Phoenix, Ariz., and at Palm Springs, Cal. In the former 

 case the tree had been without irrigation for six years and in the latter 

 193 



