Water 325 



not such changes are adaptive and are advantageous to the plant. That 

 they should be so is a plausible conclusion, and for such traits as heavy 

 cuticle it may be correct. Its general validity has been challenged by 

 a number of people, notably Maximov, who has reviewed the problem 

 comprehensively ( 1929, 1931 ) , especially as to the physiological basis of 

 drought resistance. Maximov called attention to the earlier work of 

 Zalenski (1904), published (chiefly in Russian) a quarter of a century 

 before and largely neglected outside the country of its origin. Zalenski 

 observed that the veining of the leaves in plants growing in dry, open 

 spaces was much more abundant than in the leaves of those in the shade 

 or in protected spots. These observations he then extended to a com- 

 parative study of the structure of leaves on the same tree. Here he 

 found that, as a rule, leaf structure changed with the level of insertion 

 on the tree, the structure being more xeromorphic with increasing dis- 

 tance from the root. The progressively higher leaves had smaller cells 

 throughout, smaller stomata and more of them per unit of area, greater 

 vein length per unit of area, thicker and less sinuous walls in the epi- 

 dermal cells, a greater contrast between palisade and spongy layers, less 

 intercellular space, and better developed mechanical tissue. These re- 

 lationships were later called "Zalenski's law" and were independently 

 discovered by others, among them Yapp ( 1912 ) . Zalenski's results are 

 evident in herbaceous as well as in woody plants. Some data that he 

 presents for Dactylis glomerata are shown in Table 14-1. Salisbury ( 1927) 



Table 14-1. Variations in Anatomical Elements of Leaves of Different 

 Tiers in Dactylis glomerata * 



Tier 1 2 3 4 5 



Height of insertion ( cm. ) 10.2 25.2 37.0 51.0 



Length of leaf ( cm. ) 7.1 10.3 18.5 18.0 13.2 



Breadth of leaf (cm. ) 0.30 0,35 0.54 0.52 0.45 



Length of vascular bundles 



(mm./sq. cm. of leaf surface) 371 511 557 625 626 

 Mean diameter of cells of 



upper epidermis ( mm. ).. . 0.0418 0.0294 0.0272 0.0217 0.0189 



Number of stomata in field .. . 34 42 61 80 64 



Length of stomata ( mm. ).. . 0.0434 0.0415 0.0403 0.0356 0.0384 



o 



From Maximov (1929), after Zalenski. 



found that stomatal frequency per unit area increases with the height at 

 which the leaf is borne but that the stomatal index (ratio of stomata to 

 epidermal cells in the same region) changes relatively little. This is a 

 necessary implication of Zalenski's observations. 



These structural characters are among those regarded as typically 

 xeromorphic. Zalenski, Yapp, and others, however, have explained them 



