112 Introduction to Botany. 



of division for a long time. It is a matter of common 

 observation that grass stems are easily pulled apart at the 

 nodes, and that the place of rupture is tender, succulent, 

 and sweet; these are all characteristics of regions where 

 cell division is rapidly going on. The tender bases of the 

 nodes of grass stems are strengthened by being enwrapped 

 by the bases of the leaves. 



In the leaves of grasses, and in all leaves which become 

 much elongated, or which spring from underground bulbs, 

 etc., division of the cells of the basal portion continues for 

 some time ; but leaves in general owe their growth mainly 

 to the enlargement of the cells which constitute them in 

 their embryonic condition in the bud. 



In some plants, the elongation produced by the division 

 of the cells at the apex is continued indefinitely through the 

 growing season, as illustrated by roses and morning-glory, 

 while in others the elongation of the shoot soon ceases with 

 the formation of a winter bud, as shown by the hickory and 

 horse-chestnut. 



86. Phases of Growth. — Observation 95 has shown us 

 that the region of greatest elongation is a short distance 

 back of the growing apex ; this is the region where the 

 daughter cells, produced by cell division, are increasing in 

 size. Back of this region, elongation has ceased, and thick- 

 ening of the cell walls and changes in their chemical con- 

 stitution and in the condition of the protoplasts are taking 

 place. We might, therefore, speak of three phases of 

 growth: (i) the phase of cell division; (2) the phase of 

 cell enlargement; (3) the phase of cell modification. 



87. Conditions Necessary to Growth. — The conditions 

 necessary to growth are essentially the same as those requi- 

 site to the germination of seeds, which is, in reality, simply 

 a resumption of growth. Water must be at hand in suffi- 



