CHAPTER 4 



Meristems 



In many of the morphogenetic problems which they present, plants and 

 animals are very similar. The fundamental physiological differences that 

 distinguish these two groups of living things, however, produce a number 

 of developmental differences between them. Among these, that in method 

 of growth is conspicuous. Because of their ability to synthesize food from 

 inorganic substances, plants have developed, in all forms but the smallest 

 and simplest, a body which is nonmotile and anchored to the soil or other 

 substratum. This doubtless resulted, during the course of evolution, from 

 the fact that motility in a plant is not necessary for obtaining food, as it is 

 in animals. 



The motility of animals requires that their skeletons be jointed and 

 the rest of their bodies relatively soft and plastic. Plants, however, gain 

 the necessary rigidity not by a specially differentiated skeleton but by a 

 thickening of the walls of most of the cells. This is especially conspicuous 

 in the fibrovascular system of higher plants but it occurs in other tissues. 

 The plant cell wall, because cellulose is characteristically deposited in it, 

 is a much firmer structure than the rather tenuous membrane which sur- 

 rounds typical animal cells. As a result, plant tissues themselves are also 

 firmer, save in exceptional cases such as certain short-lived floral parts. 

 As a consequence of this distinctive character, most plant cells, as soon 

 as their final size is reached, become locked up, so to speak, in a firm box 

 of cellulose. Such a cell ordinarily does not divide further, or if it does 

 its daughter cells cannot expand, so that mature plant tissue usually grows 

 no more. In almost every part of the soft-celled animal body, on the 

 contrary, growth occurs not only during development but in the restora- 

 tion and repair of tissues throughout the life of the individual. It should 

 be remembered, however, that under certain conditions a plant cell or a 

 group of cells may become embryonic again and begin to divide (p. 232), 

 setting up a new growing region. There is no doubt that most cells— per- 

 haps all— are potentially able to do this. What prevents it is not simply 

 mechanical confinement by the wall but so-called correlative factors that 

 limit each cell to the development appropriate for its particular place in 

 the organism. 



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