6o8 POPULAR SCIENCE MONTHLY. 



tain that, in any case where a wall does exist, it consists of true 

 cellulose. 



The protoplasm of a cell is usually bounded at its outside by 

 a denser hyaline layer which is very impermeable for fluids while 

 living, and thus serves as a protection against the penetration of 

 foreign or harmful substances into the cell. But this outer layer 

 appears also to be the receptive x>ortion of the protoplasm which 

 is sensitive to and transmits external stimuli. Within it is the 

 rather fluid, undifilerentiated, granular protoplasm which consti- 

 tutes the basis of the cell, and in which lie the special organs of 

 which we have spoken. This wonderful mixture of albuminoid 

 or proteid substances, which has well been called " the physical 

 basis of life," must therefore possess those fundamental properties 

 of living things, the power to assimilate, to grow, and to respond 

 to external stimuli ; and it is easy to show that living protoplasm 

 possesses all these properties. But the one which most interests 

 us just here is that of assimilation. This power of converting 

 food into its own substance, which may result in the increase of 

 that substance, or growth, seems specially to belong to the granu- 

 lar protoplasm, which may be regarded as the nutritive organ of 

 the cell. Just here we note that the food which may thus be 

 assimilated must be organic substance. It may be proteid, like 

 albumin, casein, or fibrin ; it may be a carbohydrate, like sugar 

 or starch ; it may be a hydrocarbon, such as fat or oil ; but or- 

 ganic it must be. Whence comes now the supply of food ? Plain- 

 ly, in most cases, by absorption from without. In animals the 

 solids and fluids taken in are reduced by digestion to the fluid 

 form, and are then transported to the various cells of the organ- 

 ism, to be absorbed and assimilated by them. In those plants 

 known as fungi, which can develop only on living or dead organ- 

 isms, the food materials are absorbed in fluid form, being some- 

 times first reduced to that form by the action of a ferment secreted 

 by the fungus. But certain cells of most plants have the power 

 of manufacturing their own food from inorganic materials, and 

 thus of living independently of other living things. Thus the 

 green plants bridge over the chasm between the inorganic and 

 the organic, and the life of all organized beings is practically con- 

 tingent on their life. In the granular protoplasm of some cells 

 of these plants may be found difl:erentiated protoplasmic masses 

 which contain the green pigment cMorophyll, that gives them 

 their color. First recognized early in the present century, these 

 masses were observed by Naegeli, in 1S4G, to increase by division, 

 and therefore to constitute living organs of the cells in which 

 they occur. These chlorophyll bodies possess the synthetic power 

 of recombining the elements of simple compounds obtained from 

 the air and the soil, in the presence of light, into complex organic 



