486 Minnesota Plant Life. 



beg-in again. A strong electric shock stops it forever. This 

 streaming, pellucid slime is the living substance. 



Not only in hair-cells does the living substance commonly 

 occur, but in many other portions of the plant. It is found 

 at the tips of all young roots and buds. In the green cells of 

 leaves it is always present. It will be found in certain of the 

 conducting-areas of the plant in the leaf network and in the 

 young fibers of the stem. In trees and shrubs it occurs neither 

 in the mature bark nor in the mature wood of the trunk, 

 branches or woody roots. Between the bark and wood there 

 exists, however, in ordinary trees a thin layer no thicker, in- 

 deed, than tissue paper known as the cambium. In this layer 

 of cells the living substance will be found. It occurs in all 

 spores, eggs and spermatozoids, in all very young organs and 

 tissues, and in general in all growing parts of the plant. 



Physical structure of the living substance. Years ago when 

 investigation of the living substance was in its infancy it used 

 to be described as a "structureless slime" or as "living jelly"- 

 these phrases indicating how little was really known of its or- 

 ganization. The researches, however, of the last fifteen years 

 in particular, have revealed that the intimate structure of the 

 living substance is extremely complex so much so, indeed, 

 that a new science, the science of the cell, has developed, until 

 to-day libraries can be collected in this field alone. It would 

 appear that, just as the body of a plant or animal is composed 

 of cells and their products, so the particle of living substance 

 in each active cell is itself composed of smaller bodies and their 

 products, the whole constituting an organism of extreme com- 

 plexity. It may properly be said that there are now known 

 three hierarchies of life social life, individual life, and cell 

 life. In the first of these man has his place as a component 

 body, in the second he exists as a self-sustained individual, while 

 in the third lies the living basis of his organization. 



Modern research has shown that the motion seen in a living 

 cell lying under the microscope of the observer is by no means 

 disorderly or unrelated. On the contrary the kaleidoscopic 

 changes in the living substance are now known to have pro- 

 found significance in nutrition, growth and heredity. The dif- 

 ference between the old idea and the new may be suggested by 



