gg PLANT LIFE. 



by cork, which is produced by a special layer of cork- 

 forming cells. This " cork cambium " appears very soon 

 in the life of a branch ; sometimes the epidermis itself 

 (Willow), or it may be the cells of the cortex (Elder), or 

 even cells much deeper within the stem (called the 



Fig io. -Defence, (i) Head of a green fly showing proboscis with central 

 nrobe legs with sharp claws and part of antennae (on leaves of Acer pseudo- 

 pat aAusf (2) Glandular and other hairs of Shortea cahfornica ( 3) Epidermis 

 of Acer Dseudoplatanus ; {e) cells of epidermis, (/) pal isade cells. (4) Cork of 

 WalnS-^S) oufer disorganised and {b) typical cork cells. (.) cork-cambmm and 

 (d) strengthening (collenchyma) cells below the cork. 



'^ pericycle'') suddenly change themselves into a cork- 

 producing cambium ; they then divide rapidly, and in a 

 very regular manner, so as to show the very^ even, 

 homogeneous, and continuous appearance which is 

 characteristic of cork. As soon as this cork-sheath is 

 formed, all the epidermis and other cells outside it at 

 once die away, because their food and moisture is 

 intercepted. Cork is very impervious to water and 



