70 



ORGANOGRAPHY. 



first indirectly by the medullary rays with all the lateral leaf- 

 buds ; as the latter, however, continue to develop, their connexion 

 with the central pith is cut off, as will be explained hereafter in 

 Fia 162 speaking of their structure and origin. The 

 ' parenchyma of which the pith is composed is 

 generally that kind which is known as regular 

 or dodecahedral {fig. 20), so that when a sec- 

 tion is made of it, and examined microscopi- 

 cally, it presents a hexagonal, or polyhedral ap- 

 pearance {fig. 21). When first formed, the pith is 

 commonly of a greenish colour, and the cells of 

 which it is composed are filled with fluid con- 

 taining nutrient substances dissolved in it. At 

 this time it appears, therefore, to be in an active 

 condition, and that such is the case is still fur- 

 ther proved by the fact that its cells are often 

 pitted from the deposition of secondary layers 

 in their interior. Its activity, however, soon 

 ceases, so that commonly after the first year it 

 becomes nearly dry and colourless, and its cells 

 filled with air. The pith also, then, instead of 

 forming as at first a continuous column, becomes 

 broken up at various points, so as to form ir- 

 iiranch of Walnut regular cavities in its tissue. This disruption 



{jugiansregia)Q.\xt Ya-Ay be especially seen in certain herbaceous 

 vertically to show , '' , -, . \ ■.^ . • t. • .i 



its discoid pith. plants which grow with great rapidity, as m the 



common Hemlock and others of the same family. 

 In such plants it is almost entirely destroyed, merely remaining 

 in the form of ragged portions attached to the interior of the 

 stem. In some plants, such as the Walnut {fig. 162) and Jessa- 

 mine, the pith is broken up regularly into horizontal cavities 



! separated only by thin discs of its substance. It is then termed 



= discoid. 



The diameter of the pith varies much in different plants. It is 

 generally very small in hard woody plants, as in the Ebony, and 

 Guaiacum. In the Elder it is large, and also in the Rice-paper 

 Plant {Bidymoipanax {Aralid) papyrifern). The diameter not only 

 varies in different plants, but also in different branches of the 

 same plant ; but when once the zone of wood of the first year is 

 fully perfected, the pith which it surrounds can no longer increase, 

 and it accordingly remains of the same diameter throughout the 

 life of the plant. 



The pith, as we have seen, is essentially composed of paren- 

 chyma. It also frequently contains laticiferous vessels, as may 

 be readily observed by breaking asunder a young branch of the 

 Eig-tree, when a quantity of milky juice at once oozes out from 

 their laceration. In rare cases it also contains wood-cells, and 



Fig. 



Young 



