862 



THE NATURE BOOK 



the spring day to consider the more 

 immediate cause of the bursting buds 

 and expanding leaves to be seen on every 

 side. This is nothing less than the 



WINTER. 



resiunption of the flow of sap — a process 

 which has been almost entirely suspended 

 during the last four or five months. All 

 through the winter, growth on the part 

 of the deciduous trees has been at a stand- 

 still, and only sufficient of the vitalising 

 sap has been in circulation to keep the 

 specimen from dying altogether. It must 

 never be forgotten that when the tree is 

 in its fullest state of activity, there are 

 two very distinct currents of sap — the 

 ascending and the descending flow. It is 

 not an inapt illustration to compare the 

 starting into growth of the tree to the 

 lighting of a lamp. The developing 

 leaves appear to exercise just that draw- 

 ing power upon the liquid imbibed by the 

 roots from the soil, which the burning 

 wick does upon the oil in the reservoir. 

 The course which the sap pursues in a 

 typical tree may be briefly outhned. 

 Starting at the root, the fluid is carried 

 upwards through the cells of the newer 

 wood to the upper surface of the leaves. 

 At this stage the sap is really little more 

 than plain water, combined with certain 

 inorganic matter taken from the soil. 

 The force with which the sap rushes 

 upwards is very great, and by way of 



experiment a vine stem was severed a 

 few feet from the ground in the spring- 

 time, and a bladder tied over the cut end. 

 In a short while the bladder was greatly 

 extended, and finally burst, unable to 

 sustain the tremendous pressure of the 

 sap flow. It is not until the passage of 

 the sap along the upper surface of the 

 leaves that the great change comes in its 

 composition. Exposed to the magical 

 influence of the sun and air, the fluid is 

 completely elaborated. By a process 

 which is not fully understood the in- 

 organic material taken up by the plant 

 from the soil is changed in such a way 

 that it can be assimilated by the tissues 

 of the vegetable. The sap is now very 

 much altered in its consistenc}'. A large 

 proportion of water has been evaporated, 

 and the fluid is thick and rich in nutrient 

 matter. Moreover, it will have acquired 

 certain pecuhar properties — in the case 

 of the Oak, for instance, these being of 

 an astringent nature. The course which 

 the sap now pursues will take it along 

 the under side of the lea^•es, through 



SUMMER. 



The marvellous change which three weeks of spring 

 weather brought about. 



tlu' m'W tissue, and so back to the roots 

 again. 



All the summer long the leaf lives a 

 life of intense activity, manufacturing a 

 store of food material which will be put 



