THE STllUCTURE OF A LEAF 



53 



Experiments on conifers growing under normal conditions 

 of supply give the velocity at between 5 cm.-lO cm. per hour. 

 If resistance is artificially removed, velocities of 3 m. per 

 hour have been observed. High velocities reaching perhaps 

 50 cm. per hour probably occur temporarily in living plants, 

 perhaps locally even more. There is plenty of evidence that 

 the stream often comes to a standstill or is reversed in 

 direction. Varying velocities and even reversals occur 

 simultaneously in the same plant. 



If we consider for a moment the structure of a leaf, we shall 

 find that it con- 

 sists in the first 

 place of an outer 

 single layer of 

 cells, the epi- 

 dermis. The ex- 

 ternal surface of 

 each cell has be- 

 come thickened 

 and hardened and 

 thus forms a 

 cuticle, through 

 which very little 



evaporation can j^jq, 15, Semi-diagrammatic view of a leaf in section, 

 take place. Be- showing upper and lower epidermis, the latter with 

 neath the UDPer stomata. Between these two layers the mesophyll, the 

 , f, '^1 Palisade tissue above, and the spongy tissue below, 



layer 01 epiClermal ^^^Q stomata in section show how the intercellular 

 cells comes a se- spaces communicate with the external air. 

 cond layer of 



brick-like cells, called palisade cells, arranged like a series of 

 bricks on end. Beneath these again comes a spongy tissue^ 

 whose cells, which may be three or four layers thick, 

 are separated from one another by spaces, sometimes fairly 

 large, sometimes merely slits. Then comes the lower epidermal 

 layer of cells. These spaces communicate with the external 

 air by means of little valves called stomata which consist of 

 two sausage-like cells, the guard-cells, lying face to face with 

 a spindle-shaped aperture between them which opens as a 

 result of the action of sunlight. Just within each stoma there 



