SYMBIOLOGY THE BIOLOGICAL RELATIONSHIPS OF ORGANISMS 123 



Prior to the discovery of the cellular structure of plants and animals, 

 there was some discussion and theorization as to an ultimate unit. of living 

 structure; that is, what might be the smallest part of a higher plant, for 

 example, which would continue to exist and mature. Many held that the 

 phyton or phytomer (node, i.e., node and internode) was the smallest plant 

 part which would continue to exist vegetatively and develop into a new 

 mature individual. It is however known that even smaller cell aggre- 

 gates of a plant can be induced to form a new individual, as leaf and part 

 of leaf (Begonias), bits of rhizome, of tubers and of stems. In the case of 

 the lower plants and animals the reduction can even be carried to the 

 individual cell. The Saccharomycetes are probably multicellular plants 

 in the process of formation and in this group any single cell is capable of 

 forming new cell aggregates by the budding process. In the class Spon- 

 gilla a group of a few cells will form a new sponge mass. In the lichens, 

 lower algae, fungi, and liverworts, small bits representing a few cells will 

 mature into new individuals. 



With the advent of the cell theory the immediate conclusion was 



reached that the cell represented the ultimate unit of living structure 



but as already stated we now know that the cell itself is an aggregate 



of different kinds of more or less highly differentiated living units. In the 



lower forms, as amebae, paramecia, bell animalculae, etc., the cell may be 



divided mechanically and the several fragments will each develop into a 



new mature cell. Certain plastids, nuclear chromosomes and plasmic 



granules (plasomes, chondriosomes, etc.) will live for a time outside of the 



cell, in water, in isotonic solutions and in the presence of certain active 



plant constituents (caffein, asparagin). Tissues and organs have been 



transplanted from one animal to another (skin grafting, bone grafting, 



ovarian grafting, cancer grafting, etc.) and certain tissues have been 



induced to grow in artificial culture media (epithelial cells, muscular 



tissue, etc., in blood plasm) but no one has as yet succeeded in developing 



a higher plant or animal from a single detached somatic cell, nor has any 



one succeeded in perpetuating, in artificial media, any of the living cell 



inclusions. There appears to be no difficulty in keeping groups of living 



tissue cells, which are kept in an undisturbed trophic relationship, alive 



for considerable periods of time, but to induce such cell aggregates to grow 



and to multiply by septation is apparently more difficult. Fruits, seeds, 



eggs, ova and some larval forms, will remain viable for long periods under 



natural and also under certain artificially maintained conditions. The 



cells and certain plasmic cell inclusions of the apple, the grape, the melon, 



the pumpkin, the tomato, etc., will remain alive until decomposition sets 



in or until the loss of moisture becomes excessive. Sections of fleshy 



roots, tubers, fruits and rhizomes mounted in water or in isotonic solutions 



