312 Plant Physiology 



plants are in general increased by favorable food-supply and 

 a water-content of the soil which shall not ordinarily be 

 more than from 30 to 50 per cent of soil saturation, as 

 discussed elsewhere. Under favorable conditions the 

 rate of growth is rapid; for example, the roots of corn may 

 elongate at the rate of about 1^ inches per day. In many 

 cases increase in size follows elongation, although it may 

 be coincident with this. 



Increase in the size of roots commonly involves no 

 shortening of the root-axis, yet in the rooting of certain 

 bulbs, and following the germination of a few seed, shorten- 

 ing may occur after the roots are fairly fixed in the soil, 

 thus resulting in effectively burying or sinking the storage 

 organ. The practical advantage is evident. 



180. The stem apex. - The stem apex of the flowering 

 plant shows, like the root apex, no single apical cell from 

 which growth proceeds ; instead, there is within the 

 epidermis a group of cells rather indefinite in area which 

 constitute the primary meristem. In the apical cells of 

 this meristem divisions rapidly occur, and there is also a 

 rapid extension of those somewhat older, or farther from 

 the tip. This multiplication and elongation of cells is 

 the direct cause of the observed increments of growth. 

 The epidermal layer in order to accommodate this increase 

 in growth is extended by divisions perpendicular to the 

 surface (anticlinal divisions). 



The cells of the meristem are gradually differentiated 

 posteriorly in two chief regions, - - an outer, or periblem, 

 and an inner, plerome. It is primarily within the plerome 

 that vascular tissue in seed-plants is differentiated (sec- 

 tion 186). 



