44 THE INNER ORGANIZATION OF TREES. 



of the ducts, and its cause is to be sought for in the vital 

 economy of the tree itself. The inner portion 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 necessities 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 dis- 

 appears from their cavities and we find in them neither pro- 

 toplasm nor cell nuclei. Their nitrogenous contents have 

 been expended either in the lignification of their 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, im- 

 portant and variable tissue of plants. The parenchyma cells 

 of the pith are spherical or ovoid when they are but slightly 

 united together, but more often they become more or less 

 polyhedral by reciprocal pressure. 



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



