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- 

 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 ani-T 

 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 

 on exposure to the 



air. 



Fig. 55. 



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 



