50 BOTANY OF THE LIVING PLANT 



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

 the cellulose framework (Fig. 27, A), embedding it and extending into 

 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. 27, 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. There is con- 

 siderable evidence that the transfer of metabolic materials, such as 

 sugars and amino acids, from one part of the plant body to another 

 takes place through the sieve-tubes (see Chapter VIII.). The function 

 of the nucleated companion-cells, which show great constancy of 

 occurrence, is still unknown. 



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 cells 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 cavities 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 centimeters, though sometimes con- 

 siderably greater. 



Similarly the sieve-tubes 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 technically speaking the sieve-tube also is a cell-fusion. 

 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. 



Stems of Monocotyledons. 



In the Monocotyledons both the arrangement and the structure of 

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

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

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

 bundles, but no cambium. The vascular strands are sometimes dis- 

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

 (Fig. 28). In other cases their regularity is disturbed, the largest 



