SOME MODERN VIEWS OF THE CELL. 609 



compounds which can serve the organism as food. Thus plant 

 cells which contain chlorophyll bodies differ from all other cells 

 in manufacturing their own food and in not being obliged to ob- 

 tain it from without. Since in all or nearly all plants the lack of 

 chlorophyll, when it is lacking, is due to degeneration in conse- 

 quence of the acquirement of a saprophytic or parasitic mode of 

 life, the possession of chlorophyll bodies and the consequent food- 

 forming power constitute the most real distinction which sepa- 

 rates plants from animals. Treated understandingly, this affords 

 the most satisfying response to the ever-recurring demand for a 

 statement of the differences between the two organic kingdoms, 

 although the distinction is no more an absolute one, as shown by 

 the case of the fvngi, than any of the other less important ones 

 often suggested. 



We have seen that most plant cells possess firm walls, and it is 

 little more than a decade since plants were generally believed to 

 consist of blocks of protoplasm quite shut off from each other, in 

 most cases, by the surrounding walls. The many difficulties en- 

 tailed by such belief, and the impossibility of explaining the 

 transfer of substance or the transmission of stimuli in certain 

 tissues, was the chief incentive to Gardiner's researches. This 

 author and others after him have shown that, in most tissues, and 

 especially just where they are needed to explain observed phe- 

 nomena, tiny threads of protoplasm penetrate the cell walls, con- 

 necting the protoplasmic masses of neighboring cells and forming 

 the means of communication between them. So that we no longer 

 think of the cells of a multicellular plant as isolated masses of 

 protoplasm, but as connected masses, while the intervening walls 

 give the necessary rigidity and resistance to the tissue. 



Passing now to the nucleus of the cell, we find a complicated 

 structure. Surrounded by undifferentiated protoplasm, it is 

 bounded against it by a very delicate " nuclear membrane." With- 

 in this is a loose network of somewhat solid substance, whose 

 meshes are believed to be filled by a clear, structureless fluid. In 

 this lie one or more small globular masses of a very strongly 

 refractive substance, known as nucleoli. That the nucleus is the 

 controlling organ in the more active cell processes is indicated by 

 many facts. It has been found that a cell from which the nucleus 

 has been removed is unable to grow or to form new cell wall. In 

 cells in which growth or any active process is taking place at 

 some definite point, the nucleus takes a position near to that point, 

 although thus lying far from the center of the cell. Any shifting 

 of the point of greatest activity is accompanied by a correspond- 

 ing change in the position of the nucleus. The centrospheres lie 

 ordinarily close beside the nucleus and play their chief role in 

 connection with its division, which we may proceed to discuss. 



