THE TISSUES OF THE STEM 43 



As a tube grows older a mass of callus substance is formed around 

 the cellulose framework (Fig. 27, Aj, embedding it and penetrating its 

 pores, so that finally they may be quite closed. In most plants a tube 

 which has been thus closed does not resume its function {e.g. Cucumber) 

 (Fig. 28, C). But in some cases {e.g. Vine) the autumn-formed 

 callus, which is a readily soluble carbohydrate, may be re-absorbed 

 in the spring, and the sieve-tube resumes its activity. The function 

 of the sieve-tubes is for the transfer of the less-diffusible substances, 

 such as proteids. But it is possible that other substances, such as 

 sugars, may also be conveyed by them. The function of the nucle- 

 ated companion-cells is still unknown. Judging from the constancv 

 of their occurrence it is probably an important one. 



The sieve-tubes are sometimes called bast-vessels because of the analogy 

 in development and structure between them and the vessels of the wood. In 

 both cases a number of cells fuse to form the vessel. In the wood-vessel the 

 walls separating these ce'Js are occupied by one or more large pits. As the 

 walls thicken with woody deposits these thin pit-membranes break down, 

 while the original protoplasm is absorbed. The ca^dties of the cells thus 

 coalesce into a continuous tube, which is filled in life by sap. with or without 

 bubbles of gas. They serve as open channels of transit for water with sub- 

 stances in solution. But the distance through which they are continuous 

 as open tubes is usually limited to a few centimetres. 



Similarly the sieve- tuoes originate from a number of cells usually attached 

 end to end. The terminal walls bear the sieve-plates, each plate is thickened 

 in a reticulate manner, and the meshes are styled sieve-fields, which are 

 actually individual pits. Each of these is stopped when young by a pit- 

 membrane which is perforated by fine threads. These perforated membranes 

 are then completely absorbed, so that a thick rope of protoplasm replaces the 

 fine threads. Thus the sieve-tube is also a ceU-tusion. But at maturity its 

 walls still consist of cellulose ; the protoplasts lose the nuclei they originally 

 contained, and the tube is filled with a vacuolated column of non-nucleated 

 cytoplasm, which is continuous through the open pores of the sieve. The 

 analogy of their development with that of the wood-vessels is close, but the 

 contents and the function are different. 



Stejis of ^Monocotyledons. 



In the Monocot^dedons both the arrangement and the structure of 

 the vascular strands may differ from that m the Dicotyledons, though 

 the general plan is essentially the same. The cortex in Monocotyledons 

 is reduced, but the stele is distended, containing isolated vascular 

 bundles, but no cambium. The vascular strands are sometimes 

 disposed in a simple ring round a central pith, as in Tamus or Schoenus 



