304 



HOW CROPS 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- ' j^ 



lion, milk-weed, etc. 

 These milk - ducts « 

 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 ani-// 

 mal milk, is a watery 

 fluid holding in sus- 

 pension minute gran- 

 ules or drops which 

 make it opaque.a 

 The latex often con- 

 tains the organic 

 substances peculiar 

 to the plant, acquires 

 a sticky, viscid char- 

 acter, and hardens Fig. 55. 

 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 stems of the exogenous 



