558 GROWTH, ASSIMILATION, AND ACCUMULATION 



tion phase of growth. The walls of some cells, such as those of the pith, 

 the living cells of the phloem, and most of the cells in the cortex, retain their 

 original cellulose-pectic composition indefinitely. The walls of other cells, 

 such as those of most of the xylem tissues, become lignified. Similarly suberin 

 lamellae develop in the walls of cork cells during maturation. 



During maturation cells fall into two classes, those which retain their 

 protoplasmic contents more or less indefinitely, and those in which the pro- 

 toplasm distintegrates during the process of maturation. In general the pro- 

 toplasm soon disappears from those cells in which the walls become lignified 

 or develop suberin lamellae, while cells in which such modifications of the 

 wall do not occur retain their protoplasm in an unimpaired condition for a 

 much longer period. Further structural differentiation of walls of cells in 

 which the protoplast dies, such as vessels, tracheids, and fibers, occurs only 

 by such purely physioco-chemical activities as may continue in them, or under 

 the influence of the activities of adjacent living cells. The changes occurring 

 in the heartwood of trees, for example, are a result of such processes. 



Disintegration of certain parts of cells may also occur during the matura- 

 tion stage of growth. A familiar example of this is the disappearance of the 

 cross walls between xylem elements in the formation of vessels. 



Assimilation of carbohydrates continues during all types of cell maturation 

 which involve thickening of the cell walls, but practically no proteinaceous 

 foods are assimilated during this phase of growth. In general the respiratory 

 activity of mature cells is distinctly lower than that of dividing or enlarging 

 cells. 



The end result of the maturation stage of primary growth is the construc- 

 tion of the tissues of the growing axis according to a pattern which is more 

 or less characteristic for each species. Cells become observably differentiated 

 into epidermal cells, cortical cells, sieve tubes, cambium, vessels, tracheids, 

 pith cells, etc. according to their position in the tissue mass. 



The physiological aspects of growth at root tips are fundamentally the 

 same as for growth at stem tips, but there are some important morphological 

 differences between the two. Growth of root tips has already been discussed 

 in some detail in relation to the problem of the absorption of water (Chap. 

 XVI), hence at this point we will merely summarize the principal differences 

 in growth in these two types of apical meristems : ( i ) The apical meristems 

 of stems are truly terminal while those of roots are located, with rare excep- 

 tions, between the root cap and the zone of cell enlargement. (2) The region 

 of cell elongation is usually much shorter (seldom more than i mm. in length) 

 than the corresponding region in stem tips. For this reason the entire growth 



