LEAVES 587 



than are deciduous trees, they have compensatory advantages, such as 

 lower summer transpiration, less danger from frosts in the growing 

 season, and readiness for synthetic activity at all seasons. The broad- 

 leaved sclerophylls abound chiefly in regions of winter rain, their ad- 

 vantage seeming to consist in the possibility of utilizing every day 

 suitable for synthesis, while they are also well protected from winter 

 cold and summer drought. The advantage of the delicate evergreen 

 leaf is obvious where uniform moisture and temperature prevail. 



Protective features in the cell sap and in the protoplasm. — Many plants appear 

 adequately protected from transpiration and from other dangers, although lacking in 

 such protective structures or forms of behavior as have been described. For exam- 

 ple, many plants transpire less and some much less in hot, dry weather than at other 

 times; in the Mediterranean region, transpiration is low not only in winter (as 

 would be expected), but also in midsummer, and cases are known where transpira- 

 tion in hot, dry air is reduced to one-sixth of the amount recorded in moist air. 

 Such behavior is due in part, of course, to stomatal closure and to an increase in 

 the concentration of the cell sap, but it is due in much larger part to complex 

 causes that are as yet unknown ; when most of the water has evaporated, the plant 

 enters a state of comparative inactivity, and transpiration is greatly reduced. Most 

 plant activities take place between 0° C. and 45° C, and at certain temperatures 

 (varying with the species) beyond those at which growth is checked, life itself is 

 destroyed. Death from freezing generally has been attributed to the desiccation of 

 the protoplasm incident to the withdrawal from the cell sap of water which contrib- 

 utes to the formation of ice crystals.' Many plants show remarkable resistance to 

 freezing temperatures, and it is probable that such resistance in many instances is 

 due to high osmotic pressure in the cell sap ; for example, in Phycomyces, ice does 

 not form at temperatures above —17° C. High pressure may be habitual, as in 

 many xerophytes (p. 493), or there may be changes in the pressure, accompanying 

 the temperature changes. An instance of varying pressure is seen in the numerous 

 northern evergreens, whose leaf cells in winter contain sugar instead of starch, and 

 thus have a more concentrated cell sap than in summer ; it is believed also that 

 sugar retards the coagulation of proteins which otherwise would be induced when 

 the cell salts become concentrated. The resistance of red leaves to low tempera- 

 tures also may be associated with their high sugar content. The injury occasioned 

 in early autumn or in late spring by a frost that in winter would be harmless may 

 be explained in part by the lower sugar content at such seasons. It must be ad- 

 mitted, however, that many cases of resistance to freezing or to desiccation as yet 

 remain unexplained. Experiments have shown that while there is an undoubted 

 relation between the death point and the osmotic pressure of the cell sap, it is far 

 from being an exact relation, and there are many cases in which such an explana- 

 tion is totally inadequate. Perhaps the best illustrations of such unexplained resist- 

 ance are found among the bacteria, algae, and lichens. Bacteria in the so-called 



' However, there is evidence that in many cases death may take place without actual 

 ice formation, also that ice formation does not always result in death. 



