RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 25 



grown for six years and compared from time to time as to external 

 " appearance and parts, color and dimension of leaves and flowers, 

 development of subterranean parts, etc. It was proved that a plains 

 plant when transplanted to a higher altitude acquires a certain num- 

 ber of characteristic modifications, of which some increase in- 

 definitely with altitude, and others (chlorenchyma, color of flowers) 

 reach their optimum within the limits of the altitude which the 

 species can support. The modifications of the leaves of the alpine 

 plants compared with those of the plains were found to be as fol- 

 lows : the assimilative tissue is better placed for chlorophyll func- 

 tions ; palisade tissue is better developed, either by means of longer 

 closer cells or by increase in number of rows ; chloroplasts are more 

 abundant and more deeply colored ; secretive canals, where present, 

 are either relatively or absolutely larger ; epidermal cells are usually 

 smaller and the number of stomata often greater for unit of surface, 

 especially on the upper surface. Chlorophyll assimilation and 

 chlorovaporization are greater per unit of surface. The causes of 

 the modifications obtained are ascribed to intense illumination, dryer 

 air, and lower temperatures. The first two act in the same way by 

 increasing assimilation and evaporation to produce greater thickness 

 of leaf, greater development of palisade tissue, more chlorophyll in 

 each cell, thicker cuticle and greater number of stomata per unit 

 of surface. To temperature in combination with light and dryness 

 may be attributed all protective tissues. 



Wiesner (91) carried on a number of experiments with plants 

 placed in saturated air and in darkness, but noted external changes 

 merely. 



Wagner (92) has drawn the following conclusions from a study 

 of alpine plants: 



1. The leaves of alpine plants show in every respect an unmis- 

 takable adaptation to increased assimilative activity. This is ex- 

 pressed by an increase in the size and number of palisade cells, 

 usually looser structure of the mesophyll, and the widespread occur- 

 rence of numerous stomata on the upper surface of the leaves. 



2. The grounds for the above development of the assimilative 

 tissue are to be found in : 



(a) Increased light intensities which arise because of the thin- 

 ness and dryness of the air. 



(&) Decrease of absolute carbonic acid content of the air with 

 altitude. 



(c) Shortened vegetation period. 



