120 THE PLANT 



epidermis so that the amount of Hght and heat is diminished, and the access 

 and movement of dry air impeded. While hairs assume the most various 

 forms, all hairy coverings serve the same purpose, even when, as in the 

 case of Mcscmhryanthcniuui, they are primarily for water-storage. Hairs 

 protect stomata as well as epidermal cells : the greater number of the former 

 on the lower surface readily explains the occurrence of a hairy covering on 

 this surface, even though absent on the more exposed upper side. In some 

 cases, hairs are developed only where they serve to screen the stomata. 



The modifications of the stomata with respect to transpiration are numer- 

 ous, yet all may be classed with reference to changes of number or level. 

 With the exception 'of aquatic and some shade plants, the number of stomata 

 is normally greater on the less exposed, i. e., lower surface. The number 

 on both surfaces decreases regularly as the danger of excessive water loss 

 increases, but the decrease is usually more rapid on the upper surface, which 

 finally loses its stomata entirely. It has been shown by many observers that 

 species growing in dry places have fewer stomata to the same area than do 

 those found in moist habitats. This result has been verified experimentally 

 by the writer in the case of Ranunculus sceleratiis, in which, however, the 

 upper surface possesses the larger number of stomata. Plants of this 

 species, which normally grow on wet banks, were grown in water so 

 that the leaves floated, and in soils containing approximately lo, 15, 30, 

 and 40 per cent of water. The averages for the respective forms were: 

 upper 20, lower o; upper 18, lower 10; upper 18, lower 11; upper 11, 

 lower 8; upper 10, lower 6. Reduction of number is effective, however, 

 only under moderate conditions of dryness. As the latter becomes intense, 

 the guard cells are sunken below the epidermis, either singly or in groups. 

 In both cases, the protection is the same, the guard cells and the opening 

 between them being withdrawn from the intense insolation and the dry air. 

 The sun rays penetrate the chimney-shaped chambers of sunken stomata 

 only for a few minutes each day, and they are practically excluded from the 

 stomatal hollows which are filled with hairs. The influence of dry winds is 

 very greatly diminished, as is also true, though to a less degree, for leaves 

 in which the stomata are arranged in furrows. Sunken stomata often have 

 valve-like projections of cuticle which reduce the opening also. Finally, in 

 a few plants, water loss in times of drouth is almost completely prevented 

 by closing the opening with a wax excretion. 



5. Modifications in the chlorenchym. A decrease in the size and number 

 of the air passages in the leaf renders the movement of water-laden air to 

 the stomata more difficult, and effects a corresponding decrease in transpi- 

 ration. The increase of palisade tissue, though primarily dependent upon 

 light, reduces the air-spaces, and consequently the amount of water lost. 



