FOLIAGE. 1 4 1 



dermis. (6) The skin or epidermis of the lower surface 

 which is interrupted by a small " stoma " or pore. 



This leaf is a fairly typical one, but as a rule the 

 long cells (2) or palisade cells are more conspicuous. 

 In some leaves, however, there are long cells on both 

 sides of the leaf, and in others there are very few spaces 

 between the cells. The resin-gland (3) is unusual, 

 though protective crystals or hairs are common. The 

 working of such a leaf may easily be followed by means 

 of the small arrows. The barbless show the course of 

 the water-particles. Entering the leaf by the xylem 

 tracheids, the water diffuses into every cell ; most of it 

 eventually, evaporates into the spaces between the cells, 

 and passes out by the stomata (transpiration). A 

 regular stream of water mounts into the leaf, and this 

 evaporation ^ or transpiration has a great effect in pro- 

 ducing it. The palisade cells require a continual 

 stream of water, not only to keep them distended and 

 fresh, but also to bring to them continual supplies of 

 nitrates, sulphates, potash, etc., in solution. This ascend- 

 ing sap or transpiration current may be compared there- 

 fore to a navigable river which brings them food 

 material as well as water. Part of the water, however, 

 enters the long palisade cells, and there meets the 

 carbonic acid gas particles which (small arrows with 

 two pair of barbs) have entered the stomata and 

 eventually found their way by the spaces between the 

 cells into the palisades. By the work of the chlorophyll 

 bodies, the water and carbonic acid gas are made to 

 unite, forming first sugars, and then, by the addition 

 of nitrates, etc., more complex bodies. These sub- 

 stances pass (as shown by long arrows with one pair 



^This process is not mere physical evaporation as it is controlled by the 

 living protoplasm of the cells. 



