PITTED TiSSrE. 



25 



which soon becomes a part of its wall. Tho cells keep in- 

 creasing in size, till at last the cytohlast is only a minute bo- 

 dy, imbedded in the side of the cavity, or sometimes loose in 

 the cavity. The cytoblast is sometimes absorbed after the 

 growth of the cell, and at others is permanent as in the hairs 

 of the Tradcscantia." 



The cytoblast seems to exercise the same influence on the 

 circulation in the cells, which contain thom, as the green glo- 

 bules do in those of the Chara, and like constructed cells. 

 This may be beautitully seen in the hairs of the Tradcscan- 

 tia and Cucumis. The currents receive their impulse from 

 the cytoblast, and return to it : as the cytoblust in time be- 

 comes absorbed, the current ceases. 



These two classes of circulation are well established, and 

 we are led by the most conclusive evidence that every vege- 

 table cell in its earliest state has for its type the construction 

 of the Chara, or that of the cell with the cytoblast ; and the 

 cytoblast seems to pertbrm the same functions as the green 

 globules. 



Section 2. — Pitted Tissue, 



Dotted Ducts, or Bothrenchjma (fig. 7) is formed of a series 

 y of short cylindrical cells, placed end to end, 



and in their young state may be separated in- 

 r IB ■ i I ^^ ^'^^ individual cells which compose the tubes ; 

 r !H ' Br ^"^^^ ^^^y <idvance in age the separating mem- 

 brane closing the ends of the cylinders is rup- 

 -| tured, thus forming a continuous tube ; this 

 may be distinctly seen, under favorable circum- 

 stances, in the hickory or oak, where the mem- 

 brane may be seen ruptured in some cases on 

 one side, leaving the membrane attached to the 

 other side, assuming somewhat the appearance 

 of a valve ; in others it is ruptured in the cen- 

 ter — the membrane cohering to the sides of the tube. They 

 are the largest of the vessels and are scarcely found in any 

 other situation than the wood. They are very distinct in th^ 

 beach, oak and hickory, being the largest pores observed on 

 a transverse section of these several kinds of^ wood ; but in the 

 pine, and trees of the same family, we believe they are never 

 found. This form of tissue derives its name from rows of 

 dots regularly arranged on its inner surface which are suppo- 

 sed to be grains of amylaceous substance. 



24. Under this form of tissue is usually arranged a variety 

 found mostly in the roots of plants, which appear to be spiral 

 8 



