GROWTH AND SECRETIONS. 65 



reach ? The course of growth, however, as 

 far as we can trace it, seems to be the following. 

 When a leaf bud begins to be developed, it is 

 seen to be formed of a short axis surrounded by 

 many leafy folds or scales. This axis begins to 

 lengthen ; the ascending sap is consumed by the 

 developing leaves, which separate from each 

 other by nearly equal distances, proving that 

 the shoot increases through its whole length. 

 The power of extensibility which is inherent 

 in vegetable tissue, especially when young, is 

 now probably an agent in the growth ; the as- 

 cending sap, which is partially decomposed in 

 its upward course, supplies some nutritive matter 

 to the young cells, and, it may be conjectured, 

 stimulates them to that method of increase by 

 the spontaneous formation of one cell on the 

 surface of another, of which mention was made 

 in describing the cellular tissue. The young 

 leaves now begin to perform their office, they 

 exhale water, decompose carbonic acid gas, and 

 the formation of a descending current com- 

 mences. This descending sap, depositing in its 

 course such nutritive materials as are proper 

 for the formation of wood, gradually solidifies 

 the new shoot. If the ascent of the sap be aug- 

 mented by placing the plant so that it may ab- 



