IRRITABILITY 193 



parts nearest opening buds. It is here that the cambium 

 first resumes its activity, the more and more distant parts 

 coming only successively into activity again.* It is pre- 

 cisely the parts nearest the fully expanded leaves and the 

 maturing terminal buds in which the cambium also first 

 ceases to be active. In plants which form no terminal buds 

 — such as roses, briars, etc. — the cambium continues to be 

 active so long as the temperature, moisture, and other ex- 

 ternal conditions make growth possible. The cambium cells 

 divide more or less early, and a larger or smaller number of 

 times. To a considerable extent at least this is according 

 to the behavior of the parts developing from the opening 

 buds. The living cells in rapidly growing leaves and elon- 

 gating internodes, demanding much food and water, stim- 

 ulate by this demand the earliest cells cut off by division 

 of the cambium cells to grow to such size and to take on 

 such characters that they will best conduct what is needed 

 above. If the ground is so dry that only insufiicient quanti- 

 ties of water can be absorbed, or if in the preceding year 

 only insufficient quantities of food were made and stored, 

 the parts coming from the opening buds will develop less 

 rapidly or less perfectly in size, etc., and will be able to exert 

 and will exert less of a stimulating demand upon the con- 

 ducting tissues for food and water than in a better season. 

 In this way nutrition affects everything, the formation of 

 wood as well as the development of new organs. In the 

 various living cells composing the embryonic organs in the 

 bud, the impulse to grow is given by returning favorable 

 conditions. Warmth, light, moisture, etc., stimulate the 

 cells to grow, to divide, to grow again, to differentiate, 

 etc. These cells, stimulated by purely physical influences 

 from outside themselves, develop needs proportioned to 

 their physiological activities. The needs must be met, if the 

 cells are to continue their activities, by materials drawn 

 from their neighbors. So. this demand, extending from cell 

 to cell by the osmotic transfer of nutrient solutions, pres- 

 ently reaches the living cells adjoining the conducting ele- 



* Jost, L. Beziehungen zwischen der Blattentwickelung und der Gefass- 

 bildung in der Pflanze. Bot. Zeitung, 1893. 

 13 



