204 



SECRETION AND EXCRETION 



one system are formed also by cell fusions, or sometimes by 

 outgrowth of branches from the tubes which push their way 



through intervening tis- 

 sues and fuse with other 

 tubes or branches. 



In the Urticacese, 

 Asclepiadaceae, Mora- 

 cese, most Euphorbia- 

 ceae, and Apocynaceae 

 each tube arises from a 

 single meristematic cell 

 which elongates and 

 branches, keeping pace 

 with the growth of the 

 plant, and fusing its 

 branches with those from 

 other tubes and thus 

 forming an intercom- 

 municating system, so 

 that when a wound is 

 made the milk pours 

 forth abundantly. 



The milk tubes re- 

 main living for a long 

 time and probably take 

 an active part in the 

 production of the very 

 complex latex or milk, 

 which may contain 

 plastic or food sub- 

 stances such as sugar, 

 oil, starch, proteid, and 

 aplastic or non-food substances such as tannins, alkaloids, some 

 varieties of gum, caoutchouc, resins, and salts of calcium and 

 magnesium. Some of these may be mere excretions of useless 

 substances from other tissues, and some of them may be 



Fig, 114. — Laticiferous vessels from the cortex 

 of root of Scorozonora hispanica. A, as seen under 

 low power, and B, a smaller portion under high 

 power. (After Sachs.) 



