28 



ORGANOGRAPHY. 



b. Spongiform Parenchyma, which consists of stellate cells 

 (Jig. 51), or of cells with an irregular outline produced by- 

 projecting rays, and in contact only by the extremities of such 

 rays, so as to leave large irregular spaces between them. This 

 occurs commonly in the tissue at the under surface of most 

 leaves {jig. 80, c), and frequently in the air-passages of plants. 



Fig. 50. Fig. 51. 



Fig. .50. 



Round or elliptical 



parenchyma. 



Fkj. 51. 



Spongiform or stellate 



parenchyma, 



composed of stellate cells 



with three-cornered 



intercellular spaces. 



2. Complete Parenchyma.— This includes all those forms 

 which are composed of cells in perfect contact on all sides, 

 so that no interspaces are left between them. Of this 

 there are three varieties. 



a. Begular Parenchyma. — This is formed of dodecahedral 

 or polyhedral cells, the faces of which are nearly equal. It 

 commonly occurs in the i)ith of plants {figs. 4 and 5). 



b. Elongated Parenchyma, composed of cells elongated in a 

 longitudinal direction so as to become cyUndrical, or pris- 

 matic, occurring fre({uently in the cellular tissue of Monoco- 

 tyledonous Plants {fig. 9). 



c. Tabular Parenchyma, consisting of tabular cells. It is 

 found in the epidermis and other external parts of plants {figs. 

 80 and 8 1 ). A variety of this kind of parenchyma is called muri' 

 form, because the cells of which it is composed resemble in 

 their form and arrangement the courses of bricks in a wall {fig. 



52). This variety occurs in the medullary 

 rays of wood, or, as they are commonly 

 called, the silver grain (see page 83). 



Such are the commoner varieties of 

 parenchyma, all of which are connected 

 in various ways l)y transitional forms, 

 which it is unnecessary to describe here. 

 When ordinary parenchymatous cells 

 become much thickened l)y soft second- 

 ary deposits, as in the fronds of many Alg;c, &c., the tissue 

 formed by them is called l)y some authors colU-nchyma ; or if the 

 secondary deposits are of bony hardness, as in the stones of 

 fruits, &c., Ilenfrey lias proposed tlie term sterenchyma. 



It frequently ]ia))pens that ordinary parenchymatous cells 

 become thickened by secondary deposits, in such a marmer as 



Fif,. />2. 

 Murifonn parenchyma. 



