HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 109 



ordinary periderm, all over the stem, begins after that 

 of the lenticels. Comparison of sections taken at different 

 times of the year shows that in most trees the lenticel- tissue 

 produced in autumn is relatively compact, so that the 

 lenticel is practically closed up in winter. 



124. Further Work on Cork-formation. Examine various 

 other trees and shrubs, and note that the phellogen arises in the 

 hypodermis in the majority of cases, but sometimes in the epidermis 

 itself (Willow, Apple, Pear, Jasmine, Aucuba, Euonymus, Solatium) ; 

 or in about the third layer of cortex, reckoning inwards from the 

 epidermis (Laburnum, Robinia) ; or in the pericycle (Kibes, Vitis, 

 Rosa, Ericaceae, etc. ). As a rule, the more deep-seated the phello- 

 gen the greater the amount of phelloderm produced, hence phello- 

 derra is well seen in sections of twigs of Ribes (Gooseberry, Black 

 or Red Currant). 



125. Examination of Entire Boots. A good deal 

 of the structure of the root can be made out without cut- 

 ting sections. Good material is afforded by the slender 

 roots of such seedlings as Cress, Mustard, Radish, Wheat, 

 Oats, etc. Some of these should be grown in moist air 

 sow the seeds in moist porous seed-pans, or in loose sphag- 

 num in a flower-pot or lamp chimney in the case of Beans 

 and Peas ; others on muslin tied over tumblers of water or 

 culture solution ; others in garden soil. For Germination 

 Boxes and Jars see 169, 170. 



126. General Structure of Boot. Mount in water 

 the entire roots of Cress or Mustard seedlings that have 

 grown through muslin into water. 



Note (1) the tip of the root, covered by the conical and 

 usually distinctly stratified root-cap, the superficial cells 

 of which may be seen lying loose, isolated or joined in rows, 

 and evidently in the act of becoming shed or peeled off 

 the cap; (2) the root-hairs, beginning at some distance 

 behind the root-tip as unseptate and unbranched out- 

 growths of the epidermis cells, becoming longer on being 

 traced backwards from the tip, and disappearing still 

 farther back ; (3) the dense inner tissue of the vascular 

 cylinder, running through the root ; (4) the more trans- 

 parent outer tissue or cortex; (5) the rootlets, clearly 



