231 ANATOMY AND PHYSIOLOGY OF PLANTS. 



ternal figure, losing the spiral character, and becoming straight or 

 proper vessels. 



18. The pith is the soft part situated in the centre 

 of the wood, commonly surrounded by a ring of vessels, 

 but sometimes, in part, by thickened cellular tissue. 



In the new productions of trees, the pith is found to consist of 

 oval, green, and succulent bladders, which are very similar to 

 those of the bark and wood. At the expiration of one or two 

 years, more or less, these bladders become empty, exsiccate, assume 

 a spherical shape, and finally take the consistence and the colour of 

 pith, which, in the greater number of vegetables, is of a white or 

 cream colour. 



19. The use of the pith has been a subject of critical 

 dispute among botanists, for many years. It appear*, 

 however, to be essential to the growth of other parts in 

 their young state. 



In most plants, it gradually dies away as they approach to matu- 

 rity ; and in old trees it is almost entirely obliterated. In those 

 plants which have hollow stems, the central cavity, though not filled 

 with pith, appears to be lined with it. 



20. The different species of stem, as the stalk, the 

 scape, the culm, and so on, although they do not 

 possess the exact characters we have mentioned, they 

 all partake of them in a measure, or at least, a modifi- 

 cation of them. 



THE LEAF. 



21. The leaf is a very important part of vegetables, 

 consisting of an epidermis, parenchyma, and numerous 

 beautiful and delicate vessels. 



