GROWTH 149 



This may ariso almost anywhere in the cortex or in the old bast, 

 and develops on its outer face a row of corky cells which soon die 

 and constitute the waterproof layer characteristic of bark tissue. 



Primary and Secondary Tissues. — The tissues laid down by a 

 cambium are apt to be regular in the arrangement of their cells, 

 particularly in species where all the cells are of much the same 

 size. This is due to the fact that each cambium cell has produced 

 a whole row of wood cells within and of bast cells without, and 

 that these cells are naturally arranged in a straight line along 

 the radius of the stem passing through the cambium cell which is 

 their common ancestor (Fig. 73). 



Tissues produced by a cambium are known collectively as 

 secondary tissues, and they all display in cross section this rather 

 regular arrangement of their cells. Primary tissues are those 

 which arise from a terminal growing- point, and in cross section 

 their cells are apt to be arranged irregularly. The epidermis, 

 cortex, and pith, and the first formed wood and bast, are all 

 primary in their origin. The great bulk of the wood and bast in 

 woody plants, together with the corky bark, is all secondary. 



Differentiation. — Growth is not mere increase in size but gives 

 rise to definite organs and organ systems, which are markedly 

 different in structure and function. The factors which cause 

 and direct this differentiation of the plant body as growth takes 

 place are not understood, but we know that the process may be 

 somewhat modified by various factors. Artificial removal of one 

 part of the plant, such as is brought about by pruning, will 

 stimulate the growth of the rest, and the removal of one organ 

 or organ group will often hasten the production of more organs 

 of this particular type. The development of certain organs is 

 also dependent on the fulfillment of certain rather definite exter- 

 nal or internal conditions. In most perennial plants, for example, 

 the reproductive structures (flowers and fruit) develop only after 

 the plant has succeeded in accumulating an ample supply of 

 reserve food from which they may be built. Whatever stimulates 

 very rank growth of the vegetative parts, such as an abundance 

 of water and nitrate salts, will tend to retard the development of 

 floral organs, and conditions of the opposite sort, (providing a 

 sufficient supply of reserve food is at hand), will favor reproduc- 

 tion. The amount of photosynthetic activity, governed chiefly 

 by the number of hours of simlight available per tlay, also has an 

 important influence on the appearance or suppression of the 



