DROUTH ENDURANCE 247 



stand alone this is not often the case. It is well known to growers of 

 trees that the trunks of isolated trees are often injured by the intensely- 

 hot sun of the dry midsummer, and this injury is known as "sun scald," 

 yet the details of the injury are little understood. Of course this does 

 not belong wholly to the problem of drouth resistance, and yet it is so 

 closely related to it that it is clearly one of the things needing exact 

 investigation. 



III. THE FOLIAGE SYSTEM. 



1. Thick Epidermal Walls. It is generally agreed that the thicken- 

 ing of the walls of the epidermal cells is a protective provision, and that 

 those trees whose leaves have this structure are drouth-resistant. Yet 

 it is a fact that some leaves of a contrary structure are known to be able 

 to endure degrees of dryness that are fatal to others of apparently more 

 favorable structure. Other things being equal, I think still that thick- 

 walled epidermis is favorable to drouth endurance, and yet we can not rely 

 wholly upon it. 



2. Protected Stomata. A little has been done to show us that 

 stomata differ much in their exposure to the dry air which tends to rob 

 the leaves of their moisture, but we are far from knowing enough, and 

 knowing definitely enough just to what extent stomata control water- 

 loss. No doubt a stomata that is exposed to the direct air currents must 

 lose more water than one not so exposed, but here again other factors 

 enter into the problem to such an extent as to puzzle us very greatly. 

 Then the number of stomata per square millimeter, their position on 

 one or both surfaces, their activity, their responsiveness .to light, long 

 humidity, etc., must necessarily have as much to do with their efficiency 

 in retarding water-loss as the mere matter of physical protection. 



3. Thickness of Leaf Tissue. It has often been remarked that thick 

 leaves are better able to withstand the dry air than thin leaves, and 

 there can be no doubt that in general this is quite true. Certainly it is 

 true that some plants of dry localities have thick leaves, but unfortunately 

 for the usual generalization, there are some thin-leaved plants that man- 

 age to live in these localities also. 



4. Firmness of Leaf-Tissue. Probably no structural character seems 

 to indicate more certainly a drouth-enduring ability than that of the 

 firmness or solidity of the tissues of the leaf. To a large extent the 

 leaves of xerophytes exhibit this character, while mesophyte leaves 

 have softer tissues, and hydrophytes still softer tissues. And on the 

 surface it seems to be the sam'e with the leaves of trees. Trees of dry 

 climates appear to have harder and firmer leaves, while those of moist 

 climates appear to be softer. Yet here again we are met by so many 

 apparent exceptions that we are puzzled to know whether the firmer 

 and harder leaves enable the trees to endure dry climates, or whether 

 it may not be that the dry conditions may have made the leaves harder 

 and firmer. In other words, we do not know which is cause, and which 

 is effect. 



5. More Palisade Tissue in the Leaves. Many observers have long 



