356 FALL OF THE LEAF. 



It certainly appears strange that the leaf-fall should be sometimes connected 

 with the approach of cold, and sometimes with that of hot weather. And yet this 

 is the fact. Heat and cold are only the indirect causes; the primary cause of the 

 fall of the leaf is the danger threatened to the plant by the continuance of transpira- 

 tion when either heat or cold is excessive. The danger of transpiration during con- 

 tinued dryness of soil and air scarcely requires much explanation. The conditions 

 may be summed up in a few words: the throwing oft'of the transpiring surfaces when 

 the drought commences, and the temporary stoppage of the sap-current — i.e. the so- 

 called " summer sleep " — furnish one of the best protective measures in plants sur- 

 rounded by air against excessive transpiration and withering. It is more difficult 

 to explain the connection between the fall of the leaf and the commencement of the 

 cold period. This is best indicated by some culture experiments which illustrate 

 these relations. When the soil, in which are cultivated plants with actively trans- 

 piring leaves (melons, tobacco, and the like), is cooled down to a few degrees above 

 zero, the leaves after a short time become faded, even although the temperature of 

 the air and the humidity of both soil and air are entirely favourable. By the 

 lowering of temperature in the soil, the absorbing activity of the roots buried 

 therein is so reduced that the water which is lost by transpiration from the foliage- 

 leaves can no longer be replaced. The leaves wither, dry up, turn brown or black, 

 and appear to be burnt or charred. In the ordinary language of gardeners they are 

 said to be " frozen " — frozen at a temperature above the freezing point, which 

 phenomenon is said to be due to the peculiar sensitiveness of these plants. It is 

 incorrect to speak of freezing in this case, however. The plants are in reality dried 

 up by reason of the low temperature of the soil and consequent lessening of the 

 stream of fluid up to the transpiring foliage-leaves. In regions which annually pass 

 through a long period of cold, the leaves of the plants are as liable to be dried up 

 by the cooling of the soil round their roots when winter approaches, as are the trees 

 in the catingas of Brazil when the hot dry season commences. They also denude 

 themselves of their leafy raiment as these do, since otherwise they would be 

 unable to make good the water exhaled by the leaves. When the temperature of 

 the air sinks below zero, frost ensues, and the water in the plant stiffens into ice, 

 this hastens the fall of the leaf, but it was already partially accomplished before the 

 frost set in, and where the leaves still cling to the branches, preparations are already 

 made for their detachment, which is brought about by the limitation of transpira 

 tion. It must not be concluded from this that plants foresee the approach of winter, 

 and that the preparations for the fall of the leaves result from such an intelligent 

 foresight; the phenomenon is much more easily explained on the assumption that 

 in a climate which renders necessaiy a long cessation of transpiration, those plants 

 llourish and multiply best whose natural characteristic is to follow a period of 

 energetic work by a long season of rest. The ultimate cause of this instinctively 

 adaptive periodicity is certainly not yet explained; it is as mysterious as those life 

 processes and phenomena which regularly recur at certain periods, which are perhaps 

 hastened or retarded by favourable or unfavourable external conditions, but cannot 



