304 



HOW CEOPS GROW. 



Milk Ducts. Besides the ducts already described, 

 there is, in many plants, a system of irregularly branched 

 channels containing a milky juice (latex) as in the 

 sweet potato, dande- 

 lion, milk-weed, etc. 

 These milk- ducts a 

 occur in all parts of 

 the plants, but most 

 abundantly in the 

 pith and inner bark 

 of stems and in the 

 cellular tissue of. 



roots. They often so 



completely permeate 



all the organs of the 



plant that the slight- 



est wound breaks 



some of them and 



causes a flow of latex. 



The latter, like a 



mal milk, is a watery 



fluid holding in sus- 



pension minute gran- 



ules or drops which 



make it opaque. 



The latex often con- 



tains the organic 



substances peculiar 



to the plant, acquires 



a sticky, viscid char- 



acter, and hardens 



on exposure to the air. Opium, India-rubber, gutta- 



percha, and various resins are dried latex. Alkaloids 



frequently occur, and ferments like papain (p. 104) are 



probably not uncommon in this secretion. 



Herbaceous Stems. Annual steins of the exogenous 



55 - 



