STEMS 723 



teins; even starch grains are thought capable of movement herein, 

 since they sometimes accumulate behind a wall or other obstruction, ap- 

 parently as logs pile up in a jam. The conduction theory rests more 

 on analogy than on experiment. Also related to the conduction and 

 food reservoir theories is the hypothesis that latex tissue represents 

 merely water tissue. Laticiferous plants usually are very succulent, 

 and it is possible that some of the substances associated with 

 water in latex have an effect similar to that of salts in halophytes in 

 retarding evaporation. While many laticiferous plants grow in the rain 

 forest, the majority, perhaps, are xerophytic. Euphorbia furnishing 

 many notable examples; however, milky-juiced plants are not as strik- 

 ingly xerophytic in distribution as are ordinary succulents. 



Another theory as to latex is that its coagulability is advantageous in healing 

 wounds, but this is no more a role of primary importance here than it is in blood, 

 which behaves similarly. A final theory is that latex, because of its poisonous or at 

 least unpalatable nature, protects plants from animals; mere contact with the milky 

 juice of Lactarius is said to be fatal to snails, and it is conceivable that its general 

 alkaloid content may make latex prejudicial to other animals. Many laticiferous 

 plants, however, are favorite food plants for man and for grazing animals. Further 

 experimentation is needed before the latex problem can be solved, and particularly 

 experimentation bearing on the causes underlying the formation of latex and latex 

 tubes. The presence of various kinds of plastids in latex tubes may indicate a high 

 degree of independent nutritive activity in spite of the absence of chlorophyll. 



The accumulation in stems of mucilage, oils, resins, crystals, tannins, 

 and dyes. — Ducts. — Stems, as well as leaves, may contain crystals or 

 be clothed with glandular hairs. In many stems there are ducts that 

 secrete and accumulate resins, oils, or mucilage, all gradations exist- 

 ing, especially in conifers, between these structures and internal 

 glands. Ducts originate as do internal glands, and their structural fea- 

 tures are similar, the chief difference being that they are elongated in an 

 axial direction, and are more or less continuous. In most cases there is 

 a branched and anastomosing system of continuous ducts throughout 

 the plant, though in pine needles the ducts end blindly. Often (as in 

 the pine, fig. 1039) the secreting cells are enclosed by a protective sheath 

 of thick-walled cells, interrupted here and there by permeable transfusion 

 cells. Mucilage ducts are characteristic of cycads and of many ferns, 

 and oil or resin ducts characterize most conifers and many composites, as 

 the rosin- weeds {Silphium). The mucilage tubes of certain liliaceous 

 plants resemble latex tubes, not alone in structure, but also in diversity 

 of contents since they contain protein crystals, starch, tannin, and 



