INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 119 



which contain no true green chlorophyll, as the Phycochromacese, 

 Diatomacefe, Fucacene, and Floridefe. 



This hypochlorin is the most readily combustible of all the con- 

 stituents of the cell. Its occurrence in every chlorophyll-grain is 

 subject to continual increase and decrease ; and comparative investi- 

 gations between younger and older states of the grains show that the 

 accumulation of starch advances pari passu with the decrease of hypo- 

 chlorin. A number of substances belonging to the class of essential 

 oils and their immediate derivatives, hitherto regarded exclusively as 

 jiroducts of a retrograde metamorphosis, can be proved to originate 

 directly within the ground-substance of the chlorophyll-grains. 



The result of these investigations is to show that the most impor- 

 tant function of the green colouring matter of chlorophyll depends on 

 its relation to respiration, since it acts as a regulator of this process. 

 By the powerful absorption of light of the chemically active rays the 

 intensity of respiration in the green parts of the plant is reduced in 

 the light below that of assimilation, and tlius the production of the 

 carbonaceous constituents of plants is rendered possible. This Dr. 

 Pringsheim considers to be the only function of chlorophyll that has up 

 to the present time been proved, and to furnish the explanation of 

 the well-known fact that the elimination of oxygen takes place only 

 in the green j)arts of plants, and only in the light. 



Chemical and Physical Properties of " Intercellular Sub- 

 stance."* — Understanding by this term what is called by most writers 

 the '• central lamella " or common wall of two neighbouring cells, 

 R. F. Sella has undertaken a series of investigations to determine the 

 mode in which this disappears or becomes absorbed in the production 

 of intercellular spaces, resin-passages, &c. The general results arrived 

 at arc as follows : — 



1. The intercellular substance or central lamella of plants under- 

 goes a variety of chemical and physical changes in the course of 

 development of the tissues. 



2. Its molecular constitution differs from that of the adjoining 

 cell-wall layers. 



3. The origin of the intercellular substance is either pure cellulose, 

 as in cambium, or, as in the apex of the stem, a substance in which 

 cellulose can bo detected only later, in the young permaueut tissue. 



4. Tlie intercellular substance of young permanent tissue consists 

 usually of cellulose, but in mature i)ormaneut tissue, such as often in 

 the bast, it can only be rarely detected directly ; usually the cellulose 

 undergoes a variety of chemical changes, and the intercellular sub- 

 stance hence varies greatly in its buhaviour witli different reagents. 



5. Tliese cliemical clianges fre(iuently lead to the partial or entiro 

 separation of colls previously closely united, as, for example, in mealy 

 fruits. The organic separation of cells is often merely a nieeliauical i)ro- 

 coss. Even wlieu the cells are artificially separated, as, for example, in 

 boiled potatoes, tlie breaking up of the tissue depends on a splitting of 

 the intercellular substance, and therefore on a purely mechanical cause. 



* 'Oeetcrr. Bot. Zcitsch.,' \xix. (1870) p. Illl. 



