THE AMERICAN BOTANIST 



129 



moisture-bearing tubes extend. They run in the same general 

 direction as the tubes in the bundles, but they are usually found 

 in the cortex of the plant stem and not in the central cylinder. 

 They differ from the tubes of the bundles in another respect — 

 they branch extensively and often form irregular networks in 

 the parenchyma. As to origin there are two different groups. 

 One group consists of single cells that become extensively 

 branched and push their way between the parenchyma cells 

 much as the pollen tube finds its w^ay down through the style. 

 In this case the latex has been likened to the cell sap in a single 

 immense opening or vacuole. Such latex cells are common in 

 the spurges, dogbanes, and milkweeds. In other plants the 

 lacticiferous vessels are formed by the fusion of several cells. 

 This type is found in the dandelion. In most, possibly in all 

 forms, a thin lining of protoplasm is found in the tubes. The 

 walls of the tubes, themselves, are thin and soft and, being 

 surrounded on all sides and pressed upon by the turgid cells 

 of the parenchyma, are quickly emptied of their contents when 

 the tissues are ruptured. 



Latex has been described as a watery fluid with suspended 

 granules, but it is more than that. It may contain sugar, starch, 

 salts, acids, oils, gums, resins, tannins, rubber, alkaloids and 

 proteids; in fact, nearly every substance secreted by plants is 

 likely to occur in it. For this reason latex tubes are often 

 regarded as organs for the storage of reserve food. Opinion 

 as to their use, how^ever, is divided. Some botanists regard the 

 latex as simply an execretion, others consider it designed for 

 the protection of the plant, and still others hold that it assists 

 the sieve tubes in transporting the food. It was once thought 

 that there is a circulation of latex in the tubes, but this is now 

 regarded as an error. It is quite possible that latex serves a 

 different purpose in different plants and that all the views held 

 concerning it are in a measure correct. 



