CELLULAK TISSUE. 



17 



they nifiy bo separated ami indiviiliial cells be exhibited un- 

 connected. W tlio pith of tlic cider be cut througli by a 

 sharp knile, tlie cut surlace, oven under a moderate nui«rnifier, 

 has the appearance of line lioney-conib ; but if a piece be boil- 

 ed in a weak solution of potash and water, and then gently 

 rubbed, the cells will separate, which they could not do were 

 the walls of simple membrane, and appear in the Ibrin of ex- 

 ceedingly minute vessicles, as in tig. 1. These 

 were the cells that gave the honey-comb appear- 

 ance to the pith when cut, before boiling. The 

 pressure of tlic cells on each other caused the hex- 

 agonal appearance, and when freed from pressure, 

 they assumed their natural form, that of minute 

 spheroidal bodies. This Ibrm of the cellular tissue 

 composes the pith of all plants, all the succulent part of 

 fruits, as of ajiples, melons, peaches, cucumbers, 6lc. The 

 sott part of leaves and bark and, a large part of the stems of 

 annual plants ; and in general, all the soft parts of the vege- 

 table structure, are composed ot these minute simple vessiclcs, 

 assuming'tijenerally, more or less the hexagonal appearance 

 2 when cut, as seen in fig. 2, from the slight pres- 



sure to which they are subjected. 



10. When the cells fit together by their plane 

 faces like geometrical solids forming the pulpy 

 substances, as in the above cases, it is called 

 parenchyma^ but when the vessels are elongated 

 and tapering, the ends lying over each other, 

 they Ibrm jtrosenchyma, (fig. 3,) of which the 

 hardest part of the bark is composed, and a part 

 at least of the wood and perhaps all of it, 

 11. The cellular tissue, besides the above ^ 



1 



i 



common forms, assumes a great variety of 

 appearances varying with the circumstances 

 in which it is placed. In the stalks of some 

 leaves the cells are in the form of cylinders, 

 (fig. 4.) being forced only in one direction, 

 by rapid growth. In the medullary process. 

 es they assume the tbrm of regular thin par- 

 allellopipeds. In some cases they are lobed, 

 owing, undoubtedly, to uncfpial pressure in 

 the early stage of their growth ; this frequently happens on 

 the under side of leaves. There are other kinds of cellular 

 tissue differently constituted frofn any of the j)receding. 



12. First, \.hQ fibrO' membraneous cellular tissue consists of 

 the usual cells witli a fibre coiled spirally within them, (fig. 5.) 



2* 



