amount of light that is quite sufficient 

 for their full development, even in deep, 

 dark woods. 



It must not be supposed from the pre- 

 ceding considerations that trees are to be 

 regarded as the most successful of all 

 plant types, since many factors other 

 than light enter into the problem. The 

 law of compensation, which one encoun- 

 ters' in so many fields, plays an impor- 

 tant part here. We have seen that the 

 tallest and most branched trees are the 

 best fitted of all plants to secure a maxi- 

 mum amount of light, but these very fea- 

 tures that are so advantageous here are 

 the worst possible from the standpoint 

 of protection. Among the dangers which 

 are constantly besetting most plants and 

 particularly trees, are the following: 

 exposure to undue amounts of light, 

 heat, cold, or wind, or the danger of an 

 excessive loss of water. It is perhaps, 

 an open question whether excessive light 

 is often a real danger, except indirectly ; 

 an increase of light causes an increased 

 loss of water, which is certainly detri- 

 mental. The greatest danger from ex- 

 cessive heat or wind is also due to the 

 fact that these agents increase the evap- 

 oration of water from plant surfaces. 

 The case of danger from cold needs more 

 amplification, since we have here an im- 

 portant direct influence, as well as an 

 indirect influence. The freezing of plants 

 is a well-known phenomenon, and the 

 damage that is caused b>y this agent is 

 very great indeed. Apart from actual 

 freezing it is well-known that certain 

 plants, especially those native to warm 

 climates, are often injured by tempera- 

 tures above the actual freezing point. 

 Much of the damage often attributed to 

 cold directly is in reality due to an excess- 

 ive loss of water. A familiar case of this 

 is seen in the winter killing of wheat 

 when not protected by snow, or of half- 

 hardy shrubs that are not. covered over 

 for the winter with straw. While cover- 

 ings of snow or straw may be supposed 

 to keep plants a little warmer than would 

 be the case if they were absent, their 

 chief value in this respect is in delaying 

 the access of cold or heat, and thus pre- 

 venting some of the ill consequences 

 which result from sudden temperature 

 changes. Of course a deep layer of snow 

 makes the plants beneath appreciably 



warmer. But the greatest value of snow 

 or straw in respect to winter-killing 

 results from the fact that these agents 

 greatly retard the loss of water by evap- 

 oration at a time when the frozen soil 

 inhibits the absorption of water by the 

 roots. Leaves that are exposed to the 

 chilling blasts of winter lose their water, 

 wither, turn brown and die, in a manner 

 that is precisely similar to the desiccation 

 of leaves in drought. From the preced- 

 ing statements, as well as from many 

 more that might be given, it may be con- 

 cluded that the danger of an insufficient 

 supply of water is the greatest danger 

 that plants commonly have to face. 

 Water is the most variable of the things 

 that plants need, hence it is the factor of 

 greatest consequence in the economy of 

 plants. We cannot get along to the best 

 advantage, if we are deprived of either 

 air or money ; we give more thought 

 to the latter than to the former, because 

 air is commonly present in sufficient 

 quantities, whereas money is a highly 

 variable thing. Similarly plants require, 

 among other things, water, air, light, and 

 heat, but in most conditions the latter 

 three are abundant enough for all plant 

 needs ; water, on the other hand, is a will- 

 of -the- wisp, whose distribution can scarce 

 be predicted, even in a general way. 



The dangers which have been consid- 

 ered in the preceding paragraph are 

 dangers which come to all plants in a 

 greater or less degree. We may now see 

 how trees meet these problems, as com- 

 pared with other plants. It is almost 

 self-evident that trees are the most ex- 

 posed to these dangers of all plant types, 

 by reason of their great height and their 

 manifold branching. An examination of 

 trees shows us that they are most admir- 

 ably protected from the bad effects of all 

 or most of the above-mentioned factors. 

 The severest period is doubtless that of 

 winter, and at this time most of our 

 northern trees are without leaves ; the 

 absence of leaves means a tremendously 

 smaller surface for the evaporation of 

 water. We have seen that the evapora- 

 tion of water in winter is exceedingly 

 detrimental to all plants, and particularly 

 to trees by reason of their unusual expos- 

 ure to wind and cold. Again leaves are 

 very delicate organs as compared with 

 the other organs of trees, and hence are 



