PHYSIOLOGY 



207 



3*. 



current ; as in that case, if through continued evaporation the nutrient \vat-r 

 should become too concentrated, it might act injuriously upon the plant. In marsh 

 and water plants the stomata react less promptly than in land plants (**). 



It has already been pointed out, in describing the morphology of 

 the stomata, that they are chiefly to be found on the surfaces of the 

 leaves. THE LEAVES ARE ACCORDINGLY TO BE CONSIDERED AS SPECIAL 



ORGANS OF TRANSPIRA- 

 TION (and assimilation, p. 

 214). This is also evident 

 from the extraordinarily 

 minute branching and sub- 

 division of the vascular 

 bundles in the blade of 

 the leaf. The adjoining 

 illustration (Fig. 190), 

 showing the nervature or 

 distribution of the vascular 

 bundles in a Crataeyus leaf, 

 will convey some idea of 

 the extensive branching 

 which the bundles of a leaf 

 undergo, especially when it 

 is taken into consideration 

 that only the macroscopic 

 and none of the finer 

 microscopic branchings are 

 represented in the figure. 

 By means of this conduct- 

 ing system, a copious 

 supply of nutrient water 

 can be delivered directly 

 from the roots to every 

 square millimetre of the 

 leaf. There is, however, 

 a special reason why the 

 leaves are so abundantly 

 supplied. They are the 

 actual laboratories of plants, in which, out of the carbonic acid of the 

 atmosphere and the water, and nutrient salts of the soil, the organic 



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S 



Fio. 190. Course of the vascular bundles (venation) in a 

 leaf of ' 'i-ntiii-ijiif. (From a photograph ; natural size.) 



building material of the plant-body is produced. For this reason it is 

 in the leaves that the broad expansions of tissue for the special pro- 

 motion of transpiration are found. The amount of water actually 

 evaporated from the leaf surfaces in the performance of their vital 

 functions is surprising. For instance, a strong Sunflower plant, of 

 about the height of a man, evaporates in a warm day over a litre 

 of water. It has been estimated that an acre of cabbage plants will 



