VARIATIONS IN STEM FORM 37 



it is some other color. The latex is entirely difTereiit from the sap of 

 the i)laiit and occurs in special latex receptacles. These receptacles 

 differ in different kinds of plants. In the l)l()odro<)t and in the con- 

 volvulus and soapwort families the latex is found in single enlarged 

 cells called latex sacs. These are usually arranged in longitudinal 

 rows but they are not fused. In some members of the poppy and 

 composite families the latex sacs become fused together either longi- 

 tudinally only or, especially in the composites, both longitudinally 

 and laterally. Thus a sort of circulatory system of latex vessels is 

 produced. Structures such as these that are formed by the fusion of a 

 number of cells are called syncytes. The mushroom genus Lactarius 

 also has latex vessels of the syncj-te type. Finally in the spurges and 

 milkweeds the latex vessels are coenocytes. This means that each 

 latex vessel is composed of several cells which, however, have never 

 been separated from each other by walls so that the vessel seems to 

 be a single large cell with several nuclei. It starts as an ordinary cell 

 in the embryo of the plant. This cell elongates, becomes multi- 

 nucleate (ccenocytic) , branches, and penetrates the tissues of the 

 plant like a parasite until it finally forms a sort of circulatory system 

 extending from one end of the plant to the other. 



Very little is known about the function of latex and the probability 

 is that it has no definite function. It is not characteristic of plants 

 that grow in any particular kind of environment. While a majority 

 of plants with latex are found in dry climates, many occur where the 

 water supply is adequate and some even grow in the wet rain forests 

 of the tropics. It does not appear, therefore, that the latex is of any 

 particular advantage. There is usually found in the latex some of 

 practically everything that is produced in the plant, including foods 

 of various kinds and waste materials. This indicates that the latex 

 receptacles are in a sense catch-all reservoirs into which anything 

 that is produced in excess of demand may be dumped. In some 

 species of spurge very interesting rod-shaped or soup-bone-shaped 

 starch grains are found in the latex receptacles. 



23. Variations in Stem Form.— The stems of different kinds of 

 plants vary very greatly in shape and size. All gradations are found 

 from a small fraction of an inch to 20 or more feet in diameter and 

 from less than 1 inch to 300 or more feet in length. These differences 

 are to a large extent inherited. There occur also very great varia- 

 tions in the stems of individuals of the same species which are due 

 to the external factors of the environment. A sugar maple tree 

 growing alone in the open usually has a relatively short main stem 



