THE SPRUCE FIR 



2G3 



plane as the xylem-plate. The rootlet here, as in 

 all Flowering plants, arises from the pericyele of 

 the main root. The layer of pericyclic cells which 

 divides to form the rootlet is here the second layer 

 from the outside. Tangential walls are first formed, 

 which separate off the plerome, or future central 

 cylinder of the rootlet, while the outer cells resulting 

 from the first division again divide tangentially into 



FIG. 109. Similar section of a root of Picea excelsa, showing 

 origin of a rootlet, en, endodermis of main root ; pe t 

 outer layers of pericycle ; other lettering as in Fig. 108. 

 Magnified about 150. (After Van Tieghera and Douliot. ) 



periblem and calyptrogen. The more internal layers 

 of the pericycle merely form the base of the central 

 cylinder of the rootlet, by which it is attached to the 

 vascular tissue of the main root. 



The outermost layer of the pericycle also divides, 

 and follows the growth of the young branch, round 

 which it forms a temporary envelope (s in Figs. 108 

 and 109) during its growth through the cortex. 

 This envelope is the digestive sac (see p. 171). 



