THICKENING DEPOSITS WITHIN CELLS. 



8G7 



from the stalk or the midrib of the leaf; and must then be exa- 

 mined as speedily as possible, since it loses its vitality, when 

 thus detached, much sooner than do the hairs. Even where no ob- 

 vious movement of particles is to be seen, the existence of a rota- 

 tion maybe concluded from the peculiar arrangement of the mole- 

 cules of the protoplasm, which are remarkable for their high refrac- 

 tive power, and which, when arranged in a "moving train," ap- 

 pear as bright lines across the cell; and these lines, on being care- 

 fully watched, are seen to alter their relative positions. The leaf of 

 the common Plantago (plantain or dock) furnishes an excellent ex- 

 ample of " rotation ;" the movement being distinguishable at the 

 same time, both in the cells and in the hairs of the cuticle torn 

 from its stalk or midrib. It is a curious circumstance, that when 

 a plant (such as the Anacharis) which exhibits the "rotation," is 

 kept in a cold dark place for one or two days, not only is the move- 

 ment suspended, but the moving particles collect together in little 

 heaps ; these being again broken up by the separate motion of their 

 particles, when the stimulus of light and warmth occasions a 

 renewal of the circulatory action. It is well to collect the speci- 

 mens about mid-day, that being the time when the rotation is 

 most active ; and the movement is usually quickened by artificial 

 warmth, which, indeed, is a necessary condition in some in- 

 stances to its being seen at all. The most convenient method 

 of applying this warmth, while the object is on the stage of the 

 microscope, is to blow a stream of air upon the thin glass cover, 

 through a glass or metal tube previously heated in a spirit lamp. 

 227. The walls of the cells of Plants are frequently thickened 

 by internal deposits, which may present very different appearances 

 according to the manner in which they are arranged. In its 

 simplest condition, such a deposit forms a thin uniform layer 

 over the whole internal surface 

 of the cellulose wall (probably 

 on the outside of the primor- 

 dial utricle), scarcely detract- 

 ing at all from its transparen- 

 cy, and chiefly distinguishable 

 by the " dotted" appearance 

 which the membrane then pre- 

 sents (Fig. 150, A). These dots, 

 however, are not pores, as their 

 aspect might naturally sug- 

 gest ; but are merely points 

 at which the deposit is want- 

 ing, 



FIG. 154. 



so that the 



original cell- 



Tissue of the Testa of the seed-coat of Star- 

 Anise : A, as seen in section ; B, as seen on the 

 surface. 



wall there remains unthick- 



ened. When the cellular 



tissue is required to possess 



unusual firmness, a deposit of sclerogen (a substance which, 



when separated from the resinous and other matters that are 



