76 ORGANOGRAPHY. 



their interior. Its activity, however, soon ceases, so that com- 

 monly after the first year it becomes nearly dry and colourless, 

 and its cells filled with air. The ])ith also, then, instead of 

 forming- as at first a continuous column, becomes broken 

 up at various points, so as to form irregular cavities in its 

 tissue. This disruption may be especially seen in certain her- 

 baceous plants which grow with great rapidity, as in the com- 

 mon Hemlock and others of the same family. In such cases 

 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 (fiy. 170) and Jessamine, the 

 pith is broken up regularly into horizontal cavities separated by 

 thin discs only of its substance. It is then termed discoid. 



The diameter of the ])ith varies much in difi'erent jdants. It 

 is generally very siuall in hard woody plants, as in the Ebony, 

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

 Plant {Aralia papijriferu). The diameter not only varies in 

 ditferent plants, but also in difterent branches of the same; 

 but when once the zone of wood of the first year is fully per- 

 fected, the pith which it surrounds can no longer increase, and 

 it accordingly remains of the same diameter throughout the life 

 of the i)lant. 



The pitli, as we have seen, is essentially composed of par- 

 enchyma. It also frequently contains laticiferous vessels, as 

 may be readily observed by breaking asunder a young branch 

 of the Fig-tree, when a quantity of milky juice at once oozes out 

 from their laceration. In rare cases it also contains wood- 

 cells, and in certain plants, as the large Umbellifera?, we find 

 spiral vessels in it. These however arc jn-obably only detached 

 ])ortions of the medullary sheath, separated in consequence 

 of the great horizontal distension to which such stems are liable 

 from the ra])idity of their growth. 



2. The Wood. — This is situated between the pith on its inside 

 and the bark on its outer (Jig. 163, r), and is separated into 

 wedge-shaped bundles by the ])assage through it of the medul- 

 lary rays {fig. 163, h). 'We have seen that in the first year's 

 growtli of an exogen(JUs stem the wood is deposited in the form 

 of an interrupted zoiu" immediately surrounding the ])ith (Jig. 

 16S). Tliat portion of the zone which is fir.>t developed consists 

 diit'liy of spiral vessels (figs. 168, t; 169, t; and 171, d), by 

 whieli a thin sheath is iormed, to which the name of medullary 

 slienlh is commonly ajjplied. This does not however form a 

 complete sheath to tlie pith, as its name would lead us to be- 

 lieve, but it is interru]>te(l at certain points by the jmssage 

 through it of the medullary rays (fig. 168, 7). This is the only 

 ]tart of an exogentnis stem in wliicii spiral vessels nonmiUy occur. 

 It appears to undergo less changes than any other ])art of the 

 wood, so that its sjjiral vessels may be freipiently unrolled, even 



