i3° 



PART II. VEGETABLE HISTOLOGY. 



pressure, producing cuboidal or polyhedral forms, Fig. 363 ; 

 sometimes the growth is greater in one direction than in any 

 other, and elongated cells like those in Fig. 364 are the result ; 

 or, lastly, by a more exuberant growth in two or more different 

 directions, tabular, star-shaped, or variously branching forms 

 may be produced. Cork and epidermal tissues often afford 

 examples of tabular cells. Fig. 365 represents a group of 

 stellate cells from the stem of the Pickerel-weed, Pontederia 

 cordata, and Fig. 366, a peculiar branching cell from the stem 

 of the Yellow Water-lily, Nuphar advena. 



Fig. 365. 



Fig. 366. 



Fig. 365. — Stellate cells from the stem of the Pickerel Weed, Pontederia cordata. 

 Magnified about 180 diameters. 



Fig. 366. — Branching cell (trichoblast) from the stem of the Yellow Water-lily, Nuphar 

 advena. Magnified about 75 diameters. 



When the cell-wall is first formed, it nearly always consists 

 essentially of cellulose, but it may subsequently undergo modifica- 

 tions, or become changed in appearance and chemical behavior 

 by the infiltration of other substances. These changes may be 

 of various kinds : 



1. The formation of a middle lamella, which has already 

 been described. 



2. The formation of cut in, or cork-substance. The outer 

 portion of the cell-walls of epidermal cells often undergoes this 

 kind of change, while the rest remains unmodified. As this sub- 

 stance is nearly impermeable to wear, it forms, when present, a 

 protecting covering to the plant. In the case of cork cells, the 

 entire cell-wall becomes encrusted by it, and in some instances, 

 at least, the primary cellulose wall is even changed into it. Cork- 

 substance is often called suberin, but no essential difference 

 exists between it and cutin. 



