44 THE INNER ORGANIZATION OF TREES. 



of the wood- ring, with its loose and porous structure, is 

 constructed in spring, when the greatest amount of sap is 

 needed for the nutrition of the growing leaves, shoots, and 

 flowers. Hence the wood-cells formed at this time have 

 wide cavities, and their walls are but slightly thickened. 

 They have been called by botanists ducts, and through 

 them the current of sap flows in early spring, when it is 

 most in demand. But in summer and autumn, when the 

 new leaves and shoots are fully developed, less sap is 

 needed. The outer and more compact portion of the wood 

 is formed at this time, and the wood-cells then developed 

 are adapted in their tubular capacity, to the diminished ne- 

 cessities of the tree ; their cavities are much smaller and 

 their walls thicker. It is the long, attenuated forms of 

 these cells which has led to their being called fibre-cells. 



The fibre and duct-cells die early. There is a gradual 

 cessation of their vitality, inseparably connected with their 

 formation. So soon as a cell ceases to form new cells, or 

 to develope or carry nourishing matter in it, so soon as its 

 fluid contents disappear and it becomes filled with air, it 

 may be considered as dead. This is the condition of the 

 fibre and duct-cells when fully developed ; then the sap 

 disappears from their cavities, and we find in them neither 

 protoplasm nor cell-nuclei. Their nitrogenous contents 

 have been expended either in the lignification of thei^ 

 walls, or they have been absorbed by the neighboring 

 cells. With the disappearance of this formative material, 

 the life of these cells necessarily terminates. 



THE PITH AND THE MEDULLARY RAYS. 



These are parenchyma cells ; the most widely diffused, 

 important, and variable tissue of plants. The paren- 

 chyma cells of the pith are spherical or ovoid when they 

 are but slightly united together, but more often they be- 

 come more or less polyhedral by reciprocal pressure. 



In the young stem, the pith is of considerable importance; 

 it abounds in nutritive matter, which serves to nourish the 

 young buds on its surface, and is often of a green color, 



