16 ORGANOGRAPHY. 



in the latter their tapering extremities overlap one another, and 

 become interposed between the sides of the cells which are placed 



Fig. 27. A rortinn of the frond of Nitophyllumlaceratum. a, a. Cell walls 

 b, b. Coll ten to (endochroine) of the cells.— H. B. Brady. 



above and below them {fig. 25). From this circumstance cells 

 have been divided into 'p arenchymatous and prosenchymcitoi cs ; 

 parenchymatous beinp: the^terni appliecl to those cejls whic h, are 

 piaced end to" end ; and prosenchymatous to those which are at- 

 tenuated, and overlap on e_ another w hen combined together to _ 

 jorm a tissue. Another distinction commonly observed between" 

 "parenchymatous and prosenchymatous cells arises from the con- 

 dition of their cell-walls ; thus, t hose of p arenchy matous cells 

 are usually thin and but little in crusteci; wHle th ose of prosen- 

 chymatous cei ls are more or less finekened by tb^ deposiHon upon 

 their mner surlaces ot various i nt rusting matters. The above 

 distinctions between~parenchymatous and prosenchymatous cells 

 are evident enough in the extreme forms of the two divisions, 

 but various transitional states occur which render it impossible 

 to draw, in many cases, a distinct line of demarcation between 

 them. 



When cells are so placed as to be uncombined with others, or 

 with the vessels of the plant, or but partially so, they are more 

 or less unrestrained in their development; but even in such cir- 

 cumstances, as in their combined state, the typical form is to be 

 more or less rounded. This form is, however, rarely maintained 

 as they grow older, although instances of such occur frequently 



