TRANSLOCATION OF CARBOHYDRATES 279 



the parenchyma outside the bundle can serve for the rapid 

 translocation of sugars, which here at least are obliged to travel 

 through the tissue of the bundle. 



Laticiferous Tissue 



Another type of plant structure now claims consideration. 

 In many plants there exists a system of " laticiferous tubes," so 

 called from their contents, which are frequently milky in 

 appearance. It has been held by some that these tubes, when 

 present, provide a further path for the translocation of sugars 

 from the leaf. Before giving any account of the structural and 

 experimental evidence brought forward in support of this view, 

 it may be noted that although a great many investigations have 

 been made upon laticiferous tubes, it does not yet seem possible 

 to point out what great advantage they are to the plants 

 possessing them or to say what led to their evolution. Quite 

 a number of functions have been ascribed to them by different 

 workers. 



Structure and Relations to other Tissues. — Two types of latici- 

 ferous tubes are distinguishable. In the one vessels are formed 

 by the absorption or perforation of adjacent end walls and 

 an anastomosing network results. Such vessels occur in 

 Cichoreaceae, Campanulaceae, Papaveraceae, Araceae, Musaceae 

 and some Euphorbiaceae. In the other type certain meristematic 

 cells of the embryo develop into long, branching tubes growing 

 continuously with the plant and passing among its cells. Tubes 

 of this kind are found in Urticaceae, Artocarpaceae, Moraceae, 

 Apocynaceae, Asclepiadaceae and most Euphorbiaceae. The 

 walls are usually thin or they may be comparatively thick and 

 are occasionally pitted. A protoplasmic lining, often with 

 numerous nuclei, is present, and it has been stated by Kienitz- 

 Gerloffthatin some cases protoplasmic connections exist with 

 the adjacent tissues. 



The tubes often occur in close relation to the vascular bundle, 

 being present in the pericyclic region or in the parenchyma of 

 the secondary phloem. According to Lecomte they never occur 

 in contact with sieve-tubes. In other cases they run through 

 the cortex. They follow the bundles into the leaf, where their 

 ultimate branches often turn up, pass between the palisade cells 

 and end blindly beneath the epidermis (fig. 14). Haberlandt 

 claimed to have seen a very definite relation of the laticiferous 



