50 Plant Physiology 



of cells is always resistant and compactly arranged within ; 

 but as the cells are pushed outward, becoming old and 

 sloughed-off by the continual addition of new cells, they 

 undergo gelatinization and decay. This latter process, 

 however, is important, for the gelatinization of these cells 

 doubtless acts as a constant lubricant to make easier the 

 course of the tip progressing through the soil. 



33. Structure of the root-tip. In the root-tip proper 

 there is a primordial meristem (Fig. 13) or region of rapid 

 cell division. It is often called the formative region, and 

 it is from this portion that the differentiation of tissues 

 proceeds. In some cases the meristem is relatively ex- 

 tensive, as in the pea and certain other legumes, while in 

 the example given (corn), barley, sunflower, and others, 

 it is limited to very few cells, differentiated tissues 

 extending practically to the morphological apex. 



The central portion of the root-tip is occupied by the 

 plerome or central cylinder, a columnar mass of cells, 

 most of which elongate considerably at a short distance 

 back of the tip. In longitudinal section they appear as a 

 rectangular type of cell in all that portion of the root 

 unoccupied by root-hairs, but many of these cells are later 

 transformed into the primary woody bundles. The cen- 

 tral cylinder is surrounded by a cortical portion termed 

 the periblem, made up at first of rather isodiametric par- 

 enchymatous cells. The inner layer of this periblem is 

 commonly differentiated by thicker walls to form a more 

 or less definite sheath or endodermis. The external layer 

 of the root-tip is the epidermis, also composed, in the 

 immediate tip portion, of cells generally isodiametric. 

 It is from these epidermal cells that tubular outgrowths 



