ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 769 



A typical collencliymatous cell can only contain much chlorophyll 

 when, at least at its corners, it borders a sufficiently large inter- 

 cellular space ; large intercellular spaces sometimes arising in a 

 collenchymatous tissue for the purpose of assimilation. 



Except in the comparatively few cases in which coUenchyma 

 occurs in bundles, it is usually disposed in peripheral discs or in a 

 cylinder. It occasionally reaches the epidermis, and then becomes 

 more or less chlorophyllaceous ; in other cases it is entirely driven 

 back from the epidermis, and then forms, on transverse section, an 

 unbroken ring, giving the plant its greatest possible elasticity. 



When the corners of the subepidermal collenchymatous cells are 

 considerably thickened, this thickening often extends only to the 

 peripheral and not to the radial walls of the cells. If, in this case, 

 the cells are of nearly equal size, and their arrangement regular, so 

 that the thickened spots of the walls are in apposition, the collen- 

 chymatous substance then forms two or more concentric cylinders. 



The mechanical function of coUenchyma is not confined to mature 

 organs, but is exercised also in those which are still growing, forming 

 the " stereome " capable of extension. In the motile cushions of 

 many leaves the stereome, then consisting partially or entirely of 

 sclerenchyma, occupies a central position. It also occurs, more or 

 less strongly developed, in growing tendrils, and in the twining inter- 

 nodes of climbing plants, as in monocotyledons, where it is otherwise 

 rare. 



CoUenchyma occupies, then, a position second only to the scleren- 

 chyma in facilitating elasticity, this being largely dependent on its 

 hydrostatic properties. 



The writer considers coUenchyma, in its typical form, fully 

 entitled to be regarded as a distinct kind of tissue, not only because 

 it is clearly distinguished from the most nearly related kinds by its 

 anatomical peculiarities, but because its cells unite more fully than in 

 any other tissue the two antagonistic characters — the tendency to 

 decided thickening of the walls, and at the same time the tendency 

 to be very loosely united with one another. 



Chemical Nature of the CeU-nucleus.* — E. Zacharias has de- 

 termined the nucleus of the cells of plants to consist of a peculiar 

 substance differing chemically from the surrounding protoplasm, and 

 identical with the nudeine, of which Miescher states the nucleus to be 

 composed in the animal kingdom, and to which he assigns the 

 formula C29H49NgP3022 in the case of animal spermatozoa. The pro- 

 perties of nucleine are as follows : — It is attacked only with great 

 difficulty by the gastric juice ; it is nearly insoluble in water, quite 

 so in dilute mineral acids, but easily soluble even in very dilute 

 solutions of caustic alkalies, ammonia, concentrated nitric acid, and 

 fuming sulphuric acid. In the freshly precipitated condition it is 

 soluble in soda and sodium phosphate ; solution of sodium chloride 

 transforms it into a swollen, coherent, tough jelly ; iodine colours it 



* Bot. Ztg., xxxix. (1881) pp. 169-76. 



