THE LEAF 



T 



in Hakea (Fig. 50). Additional control over evaporation of water 

 is gained by this means. 



Seeing that the epidermis serves for protection, and for regulating 

 the ventilation of the leaf, both functions of a secondary character, 

 it seems clear that the mesophyll is the tissue of prime importance. 

 The cells of the palisade-parenchyma 

 are oblong in form, and each is bounded 

 by a thin cellulose wall, which is 

 rounded off at the angles so as to pro- 

 vide the intercellular spaces (Fig. 48). 

 The cell-wall is lined internally by a 

 film of cytoplasm, within which is a 

 large vacuole filled with sap. The 

 nucleus maybe suspended in the middle 

 of the cell, but more frequently it 

 is embedded laterally in the cyto- 

 plasm. The most marked features of 

 these cells are the chlorophyll-corpuscles, 

 or chloroplasts, which are discoid in 

 form and of a full green colour. They 

 are always embedded in the cytoplasm, 

 and are as a rule so placed that one 

 flattened side faces to the cell-wall, 

 the other to the central vacuole. The 

 cells of the spongy-parenchyma re- 

 semble the palisade cells in all essential 

 points except in their form, which is 

 very irregular, and in the fact that 

 their chloroplasts are fewer (Fig. 45). 



FIG. 51. 



Varying positions taken by chlorophyll 

 grains in the cells of Lemna trisulca under 

 illumination of different intensity. T, in 

 diffuse daylight. S, in direct sunlight. 

 N, at night. The arrows indicate the 

 direction of the light. (After Stahl.) S. 



As the cytoplasm has been seen to be 

 capable of movement, it can alter the 

 position of the chloroplasts embedded in 

 it. Apart from any other disturbing cause, 

 they collect at the cell-surfaces adjoining 

 the intercellular spaces. Their movements are regulated by external con- 

 ditions, of which one is the aeration of the cell. Another, and apparently a 

 stronger influence is light. In diffused light they place themselves so as 

 to present their flattened surfaces to the incident rays (T, Fig. 51) ; in 

 intense sunlight they present their edges, and so protect themselves from 

 its harmful effects (S, Fig. 51). 



The origin of the chloroplasts is primarily from the plastids of the embryonic 

 cells, which enlarge and assume a green colour. These plastids multiply by 

 fission, and it is easy to see that the chloroplasts do the same. Comparison 



