846 A TEXTBOOK OF THEORETICAL BOTANY 



Apical Growth and Meristems 



We have seen above that plant growth may be either apical or inter- 

 calary. The latter is, however, except in rare instances (leaves of Welwitschia) 

 only temporary, while apical growth is conterminous with the life of the plant 

 and may be in fact continuous in a uniformly favourable climate. This 

 concentration of growth into a small, permanently embryonic area is one of 

 the great differences between plants and animals. In a young animal growth 

 is general and goes on until a specific size is reached, which is not w^idely 

 different between individuals ; at this stage general growth ceases and we 

 say that the animal is mature. A plant, however, is never mature in this 

 sense. Apical growth persists as long as the plant lives, so that the size 

 reached by a given individual is conditioned only by its age and its 

 environment. 



When we examine the apex of any growing shoot we see that it is occupied 

 by a cluster of young leaves, the outer of which more or less closely enfold 

 the inner. This is the apical bud. If growth for the year has ceased the 

 bud will be encased, not with young leaves but with hard scales. This is a 

 winter bud, which will remain dormant until next season. By the time 

 winter comes the rudiments of next year's leaves will have been formed 

 within the scaly covering, ready for rapid development in the spring. Some 

 plants, especially trees, have all the leaves for the season thus prepared before 

 the season begins {e.g., Horse Chestnut), and even where this is not the case 

 few of them continue to produce new leaves after June. They then form 

 bud scales, within whose protective covering the preparation of leaves for 

 the following season continues until October or November. 



If we dissect a bud in either the active or the dormant state we see that 

 the young leaves get progressively smaller as we go imvards, becoming finally 

 microscopic at the centre. If we now examine microscopically a median 

 longitudinal section of such a bud, we see that at the centre lies the true apex 

 of the stem, a more or less dome-shaped mass of small cells, on the flanks 

 of which the leaf rudiments arise successively as small protuberances. 

 This is the growing point or punctum vegetationis, and the tissue of which 

 it is composed constitutes the apical meristem (Fig. 837). 



This polar concentration of growth at the apices of stems and roots is a 

 verv' striking feature of plant organization. To trace its origin we should 

 probably have to go back to the primitive cell-filament from which the axial 

 structure of higher plants has been evolved, but there is little doubt that 

 it is an expression of a polarity which is inherent in every part, indeed in 

 every cell of the plant body. Each cell has physiologically a top and a bottom, 

 a front and a back, a right side and a left, and though we can see nothing to 

 suggest this in a normal tissue cell, it may come out clearly during the 

 regeneration of tissues after a surgical operation. 



Whatever its origin, let the fact suffice us that here in the apex are pro- 

 duced, from the meristem cells, all the organs of the plant. The process of 

 their development we call their ontogeny, and the conditions of their ontogeny 



