1891.] MICROSCOPICAL JOURNAL. 207 



Tiitrodnctioii to Elementary Biology. 



By henry L. OSUORN, 



HAMLINB, MINN. 



Part IV.— Tissues. 

 ( Continued from page 175.^ 



The Unit of Bodily Structure. — Cells in all multicellular ani- 

 mals or plants are the actual units performing the work of whatever 

 kind done by that being. We use language, ordinarily, in a manner 

 which obscures this fact, for we say that the dog runs, or the bird eats, 

 or the potato produces starch, the tree produces fruit, and the rose pro- 

 duces flowers. The ambiguity in this case is parallel with that where 

 we say that the sea produces fish, the town produces shoes, and a dis- 

 trict produces wheat. Strictly speaking, the ocean does not produce 

 fish, but fish perform that act; nor does a town produce shoes, but the 

 shoemakers do it. To come back now to the dog ; when we say the 

 dog jumps we think of the mental aspect of the act and the result, and 

 ignore the muscular and nervous phenomena involved. To make the 

 jump mean what and all it is, we must push our enquiry back of the 

 obvious change of position, to the causes as we can know them by 

 studying dog-structure, and this will bring us to nerve and muscle. Both 

 inference from analysis of the actions of animals and plants and direct 

 examination of all physiology, when it is possible, make it certain that 

 all activity in living bodies depends on the protoplasm in its various 

 component cells and is an aggregate of work. Our present enquiry 

 will be into tissues, the basis of the varied activities of the higher animals 

 and plants. 



Tissue as Cell- Aggregate. — We have learned from yeast, Proto- 

 coccus, Spirogyra, Amtrba, Vorticella, and others that cells divide, 

 and thus propagate their kind. In these cases the cells are separate more 

 or less completely from each other and thus each has an equal chance to 

 exercise its various powers of motility, sensation, metabolism, and 

 reproduction. Cells keep on dividing, but probably not without limit, 

 for it now seems likely that conjugation must occur in many cases, at 

 least to rejuvenate the flagging powers of cells which have been repro- 

 ducing by fission for a long period, or they will lose the power and finallv 

 die of old age. This point, however, is disputed, and there are those 

 who positively declare for the absolute immortality of all protozoa. 

 The consequence of fission on the part of cells, located so that they can- 

 not separate, would be the heaping up of piles of cells, all more or less 

 alike. Such a state of things can be observed in many developing eggs 

 of animals and plants, and such an aggregate of cells is a tissue. In 

 many of the lower plants and animals, only a small number of tissues 

 enter into the structure of the body. Thus, for example, in Chara and 

 the seaweeds, also in the sponge and Hydra the body of the adult plant 

 or animal is composed of only a few tissues, but these of vastly nu- 

 merous cells. 



Tissues with different Functions. — The diflerent cells of any 

 single tissue are all alike, both in shape and in function, and are un- 

 like those of other tissues. The consequence of the aggregation of simi- 

 lar cells is that the minute results of the action of one cell are multiplied 

 by the number of cells into an important total. Thus, for example, sup- 



