218 MIAUJTE ANATOMY OF FLOWERIXG PLAXTS 



period of active locomotion do tliey settle down and 

 become invested with a membrane. This fact, among 

 others, shows the essential independence of protoplasm in 

 cells, and the subordinate role of the wall. 



The wall is a product of the protoplasm. New walls 

 are formed by the conversion of a jjortion of the proto- 

 plasm into the substance of the wall. In 

 young cells, and many old cells, this sub- 

 stance is cellulose, cliemically resembling 

 starch. It is a regular occurrence that 

 in certain of the cells of the plant body, 

 the protoplasm becomes at lengtii wholly 

 converted into wall, when, of course, the 

 life of these particular cells is at an end. 

 In the later phases of this process, the 

 depositions may take a form differing 

 chemically from cellulose. We have, for 

 instance, in wood cells, lignified walls; in 

 cork cells, walls containing a fatty sub- 

 stance called suberin. Modified walls of 

 these sorts have physical properties differ- 

 ing' from tliosc of cellulose. For exam- 

 pie, the suberized walls of coi'k resist the 

 entrance of water, whereas the cellulose 

 of pith and the lignified walls of wood 

 take water into their pores readily. 



Walls are seldom, or never, evenly 

 thickened when the depositions are con- 

 siderable, but certain areas remain thin, 

 even after the completion of the thicken- 

 ing process. Or the greater part of the cell wall may fail 

 to thicken, and then the depjositions take the form of 

 raised markings on the interior of the walls. Examples 

 are the annular and spiral ducts (Fig. 371). 



505. Changes in the shape of the cell. — The cells of the 

 growing tips of the stem and root, and young and actively 

 dividing cells elsewhere, are, in general, nearly isodiamet- 

 rical. Subsec[uently, many of these cells liecome greatly 



366. Wood fibers in 

 longitudinal 

 section: a, 

 part of the 

 wall showing 

 face view of 

 pits ; b, the 

 pits in section. 



