164 



PLANT LIFE. 



crystals, hard-walled stone-cells, and collenchyma are 

 •chiefly found (see Chap. V., p. 97, Defence). 



The places in which food material is stored up are, 

 as has been already stated, Xh^ pith, cortex, and medullary 

 i^ays. After the fall of the leaves, a continual chemical 



Fig. 21. — Longitudinal Skction of Stem (Sparmannia). (^/) Epidermis 

 with one cell formins; a hair. (Cx) Cortex, consisting of {Co) strengthening 

 Collenchyma, {Cr) Crystal cells, {Eri) Endodermis. {.Pe) Pericvcle, {Ph) 

 Phloem, (O Cambium, {X) Xylem and {P) Pith. 



change appears to go on within these cells, of which the 

 nature and purpose is not understood. The most 

 important point to remember is, that the tree lays up a 

 store of food for the winter, generally of vegetable fats 

 and oils, though sometimes of starch. These reserve 

 foods are, on the approach of spring, transformed into 

 sugars, and other substances which are able to diffuse 

 readily through the cellwalls, and travel up to the 

 developing buds. Some of these changes appear to be 

 connected in some way with the light. It may be that 

 they require protection from some injurious effect of 

 light. It is this that, probably, explains the beautiful, 

 if inconspicuous, colours of the twigs of trees. These 



