CELLULAR TISSUE. 



11 



Fig. 2. 



from pressure, they assumed their natural form, that of minute 

 spheroidal bodies. This form of the cellular tissue composes 

 the pith of all plants, all the succulent part of fruits, as of apples, 

 melons, peaches, cucumbers, &c. The soft part of leaves and 

 bark, and a large part of the stems of annual plants ; and in 

 general, all the soft parts of the vegetable structure are com- 

 posed of these minute simple vesicles, assuming 

 generally more or less the hexagonal appearance 

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

 sure to which they are subjected. 



10t 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 form pro- 

 senchyma (Fig. 3), of which the hardest part of the 

 bark is composed, and a part at least of the wood — 

 perhaps all of it. 



1 1 . Cellular tissue assumes a great variety of forms, 

 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 medul- 

 lary processes they assume the form of 

 regular, thin parallelopipedons. In some 

 cases they are lobed, owing, undoubtedly, 

 to unequal pressure in the early stage of ^ 

 their growth ; this frequently happens on woody fibers 

 the under side of leaves. mU nH™ e ' 



12. The appearance of cells is very dif- 

 ferent in different circumstances. They sometimes appear dotted, 

 as though pierced with numerous pores. This is occasioned by 



Fig. 7- 



Pith of the Elder 



Fig. 3. 



Cells. 



the deposition of an internal layer, which is not continuous, and 

 when this layer is wanting, it gives, by transmitted light, the 



What does this form of cellular tissue compose? — 10. What is paren- 

 chyma? What prosencbyma? — 11. What are some of the forms that cells 

 exhibit ? — 12. What appearances do cells assume ? What is the cause ? 



