246 



STRUCTURE AND VITAL PROCESSES OF PLANTS 



2. The Vascular Bundles. 



These consist of two parts: an inner part made u,p of strong, 

 woody vessels, and an outer part made up of both soft and hard 

 bast cells. 



The stems of older (dicotyledonous) plants are stronger than 

 those of young plants. While they grow in girth, their woody 

 vessels increase in number so that by and by the woody parts 

 of the several vascular bundles coalesce and form one woody 



body enclosing some of the pithy cellular 

 tissue. Similarly the librous outer parts 

 of the different bundles unite into one 

 cylinder, generally denoted bark. Between 

 these two tissues there is a tender tissue, 

 the cambium cylinder, which like the tender 

 tissue in the growing point of the tip of 

 the stem and of the root is capable of 

 forming fresh cells by division. And, 

 indeed, it is due to the activity of these 

 cells that stems grow in girth: new cells 

 are added from the cambium cylinder both 

 towards the interior cylinder of woody 

 vessels and towards the exterior cylinder 

 of rind. (See also page 109.) 



The tibro-vascular bundles of mono- 

 cotyledons having no cambium, these plants do not increase in 

 uirtli. 



Fig. 226. -Section of 

 the trunk of a Mango tree 

 showing the light albur- 

 num below the bark, the 

 dark heart-wood in the 

 inner layers, the pith in 

 the centre, and medullary 

 rays stretching from the 

 bark into the centre of 

 the wood. 



3. The Channels for the Ascending and the 

 Descending Sap. 



(a) The Ascending" Sap — The water-current cannot ascend 

 in the heart-wood, the cells of which arc generally impregnated 

 with waste products, such as gum and tannin, and therefore im- 

 permeable. The dark heart-wood is tlius made durable and 

 resists the attacks of fungi which would otherwise cause the 

 wood to decay. Tliis property makes that part of the wood not 

 only useful for the carpenter, l)ut also strong enough to support 



