192 PLANT PHYSIOLOGY 



phenomena almost or quite coincide with the formation of 

 annual rings. "Spring wood" forms (seep. 123) when sap- 

 pressure is greatest, when the buds open and the leaves 

 expand, when there is a sudden extension of the surface 

 from which water will evaporate. At this time water must 

 be abundantly supplied to the parts just emerged from the 

 bud so that the new cells may expand to their proper size ; 

 food must be furnished these growing parts so that new 

 cells and new protoplasm may form and the parts may con- 

 tinue to increase in size. When the buds unfold there is an 

 immediate and great demand upon the conducting tissues, 

 but as the parts increase in size and weight, the mechanical 

 strength of branchlets, branches, and stem must increase 

 also. With an increasing weight each spring and early sum- 

 mer there is an annually increasing mechanical strain upon 

 the tree or shrub. This increased strain is yearly met by 

 increased strength, and this is contributed largely by the 

 "autumn wood." 



The time during which the cambium cells give rise by 

 division to new cells differentiating into wood and bast ele- 

 ments is much briefer than the season during which growth 

 is apparently possible. According to Jost, * the greater part 

 of the increase in thickness of stems and branches takes 

 place in May and June ( in Germany ) . This indicates that 

 the activity of the cambium cells and of their immediate 

 derivatives is controlled by influences outside of themselves. 

 These influences are doubtless many, but we may distinguish 

 some of them at least. 



The young parts growing and developing from opening 

 buds in the spring need much food and water, and they 

 certainly transpire greater or less quantities of water-vapor. 

 There is at this time an especially great and a fairly steady 

 demand upon the conducting tissues for both food and 

 water, not so much for transpiration, perhaps, as for 

 growth in the full sense of the word; for food so that new 

 protoplasm may be formed, for water so that it may prop- 

 erly expand. This demand would make itself felt first in the 



* Jost, L. Beobachtungen iiber den zeitlichen Verlauf des Dickenwachs- 

 thums der Baume. Ber. d. Deutsch. Bot. Gesellsch., Bd. X., 1893. 



