EXOGENOUS STEM. PITH. 



39 



rays being at the same time continued from within outwards. This is 

 represented in fig. 98, where (1, 1) indicates the section of the stem of 



98 ter. 



the first year's growth (the letters referring to the same part as in figs. 

 96, 97); and (2) shows the interposed zones of the second year, by 

 which the diameter of the stem is increased. 



75. The Pith, or the central part of a dicotyledonous stem, is com- 

 posed of cellular tissue, which is developed in an upward direction, 

 the cells diminishing in size towards the circumference, and being often 

 hexagonal. In the young plant it occupies a large portion of the stem, 

 and sends cellular processes outwards at regular intervals to join the 

 medullary rays (figs. 94, 95, p). The pith has at first a greenish hue, 

 and is full of fluid, but in process of time it becomes pale-coloured, 

 dry, and full of air. These changes take place first in the central cells. 

 Sometimes the pith is broken up into cavities, which have a regular 

 arrangement, as in the Walnut and the Jessamine; it is then called dis- 

 coid or disciform (S<Wo?, a disc, from the circular partitions). At 

 other times, by the rapid growth of the outer part of the stem, the 



Fig. 98. Vertical section of a branch of Acer campestre, two years old, where (1, 1) indicates 

 the portion formed the first year, and (2) that formed the second. The letters as in figs. 96 and 97. 



Fig. 98 bis.- Certain parts of the preceding magnified in order to show the structure of the 

 vessels and cells, as well as their form and direction. Fig. 98 ter. A portion of a porous vessel 

 magnified. 



