VASCULAR BUNDLES 87 



which are of great use, e.g. those in the leaves of the 

 AGAVE make * Aloe-fibre '. 



(4) Long soft tubes, not continuous, but connected 

 end to end through small holes in the terminal walls, 

 which look like the holes in the rose of a watering can 

 or a sieve. For this reason these are called sieve-tubes, 

 and it is along them that the food-stuffs manufactured 

 in the leaves, pass to the other parts of the shoot and 

 roots. 



(5) Narrower closed tubes of the same length, with 

 pointed ends. They accompany the sieve-tubes and are 

 therefore called companion-cells. 



(6) Short elements, more or less cubical in shape, 

 like the material of the ground-tissue between the 

 bundles, and called parenchyma. 



The vessels, tracheids, fibres, and some of the 

 parenchyma are made of the specially hardened kind of 

 cellulose, we call wood ; the sieve-tubes, companion- 

 cells and most of the parenchyma, of unaltered cellulose. 



These six main classes of elements, individually too- 

 small as a rule to be seen except through a microscope* 

 make up the vascular bundle, the sieve-tubes and com- 

 panion-cells, collectively called the phloem, being always 

 on the side nearer the outer surface of the stem, or the 

 lower of the leaf, the vessels and tracheids on the inner, 

 or the upper side in tne case of a leaf. 



Separate bundles can also be seen in a very few 

 dicotyledons, e.g. the Pepper, and ARISTOLOCHIA (in 

 the latter the vessels are so large that they can be 

 easily made out with the naked eye), and as a matter 

 of fact in all when quite young. But not in older stems 

 and roots, for in a very short time, generally before 



