STORAGE AND TRANSLOCATION 287 



in this region, and, as already mentioned, there is evidence to show 

 that some upward movement of stored foods also occurs in the 

 phloem. 



Latex. — In some families, translocation and storage take place 

 partly through the latex system. In the Euphorbiacese (spurges), 

 Asclepiadaceae (milkweeds), and the Moracese (mulberries and 

 related forms), the latex elements consist of separate cells that 

 may branch and run throughout the entire plant, reaching a 

 length of many inches or even feet; while in the Papaveraceae 

 (poppies) and many of the Composite (lettuce, dandelion, etc.), 

 the latex vessels form a system of branched tubes composed of 

 cells which have fused together, making a complex communi- 

 cating system between the leaves and the roots. But, whether 

 the cells are separate or fused, they approach very near to the 

 food-manufacturing cells of the mesophyll and are thus well 

 placed to collect the elaborated products of the leaf. 



In the latex tubes, the protoplasm forms a very thin layer lin- 

 ing the cells and the vacuole, which contains the watery solution 

 of gums, alkaloids, oils, etc., called latex. Some latex is colorless, 

 but more commonly it is colored as in the bloodroot (Sanguinaria) 

 or a milky color as in the milkweed (Asclepias), poppy {Papaver), 

 and spurge (Euphorbia). Commercially latex may be very im- 

 portant as in the poppy, where it is the source of the opium alka- 

 loids, and in the rubber plant. 



Whether the latex is a storage reservoir, a waste reservoir, or 

 primarily a conducting system, has not been definitely decided. 

 Some of the products seem to be waste products; others such as 

 the oils and sugars are foods, which in some cases make up as 

 much as 30% of the contained dry matter. In some plants the 

 chief survival value of latex may be in its protective function in 

 that (a) the disagreeable taste and sticky nature of the latex 

 make the plants undesirable forage, or (b) the alkaloids con- 

 tained render the plants poisonous to insects and higher animals. 

 In the deadly upas tree (Antiaris toxicaria) of the tropics, the 

 latex is very poisonous and is used by the natives to poison their 

 arrows. Furthermore, the coagulating properties of the latex of 

 some plants when exposed to the air may be of value in the clos- 

 ing of wounds. In xerophytes, latex may aid in the regulation of 

 the osmotic pressure and help to retain water within the plant. 

 In still other cases, as mentioned above, it may be a food reser- 



