550 GROWTH, ASSIMILATION, AND ACCUMULATION 



in Chap. XXIX, although obviously growing, continuously decreases in total 

 dry weight. Even when seeds germinate in the light the total dry weight of 

 the plant decreases for a period prior to the initiation of photosynthesis in the 

 developing seedling. Similarly, for a short time in the spring, when the buds 

 of woody plants resume growth, a slight decrease occurs in the total dry weight 

 of the plant. Likewise an actively growing plant steadily decreases in total 

 dry weight during the night hours. Such examples, however, only obscure the 

 crucial fact that the growing region invariably increases in dry weight during 

 the process of growth. In such examples as those cited assimilation of foods 

 in the growing regions is proceeding at the expense of accumulated foods, and 

 the high respiratory rate which is also an accompaniment of growth results in 

 the oxidation of foods and the resultant loss of total dry weight. Even under 

 such conditions the plant is increasing in terms of assitnilated dry weight. 



Meristems. — In general growth is initiated in tissues in which cell divi- 

 sions can occur. The primordial tissue of every growing region is a meristem 

 which, loosely defined, is a tissue in which some or all of the cells possess the 

 capacity of cell division. While it is evident that a meristem must be an in- 

 tegral part of every growing region of a plant, its role in the growth process 

 is a restricted one, since it only sets in operation the first readily observable 

 step in the complex series of processes which are usually included under the 

 term of growth. 



Meristems sometimes arise de novo from living cells of the plant but most 

 such regions present at any time in the body of a plant represent the current 

 generation of an unbroken lineage of meristematic cells extending back to 



the cell usually a fertilized egg cell — from which that plant body originated. 



The important meristems of which this is usually true are the apical meristems 

 at stem and root tips and the cambium. Differentiation of the initial root 

 meristem, the initial stem meristem, and the cambium occurs very early in 

 the ontogeny of any individual plant. Determination of the general organiza- 

 tion of the plant body — ^which is a basic and integral phase of the growth 

 process — thus occurs very early in its life history. 



Growth which is initiated in the apical stem and root meristems is called 

 primary growth. Primary growth results in the construction of the primary 

 tissues of a plant, accounts for all increase in length of the plant axis at both 

 stem and root tips, results in the development of the branching system of the 

 stem and roots, and is responsible for the production of lateral appendages 

 such as root hairs, leaves and floral parts. 



In many species the primary tissues constitute the entire plant. This is 

 true of the ferns and their allies, and of most monocots. In gymnosperms 

 and most dicots, however, stems and roots not only grow more or less con- 



