Minnesota Plant Life. 497 



tissue, the bast, the cork, the milk tissue and the skeleton tissue 

 will have become developed with its own structural pecul- 

 iarities. 



In growing organs the cells live together somewhat after the 

 manner of individuals in a community. They influence each 

 other in a variety of ways by mutual pressure, by contact, by 

 transfers of solutions, by propagation of impulses and even 

 by direct interchange of living substance. In growing tips 

 the partitions between the cells often have extremely minute 

 perforations through which the protoplasm is continuous from 

 cell to cell. By this continuity the tip remains an organic unit, 

 and every plant organ, from the earliest rudiment, must be 

 regarded, not as built up, cell upon cell, in the manner that 

 bricks are laid in some edifice of human construction, but 

 rather, as an elaboration of living substance growing under the 

 restraints of physical and chemical law, under the ever present 

 influences of the surroundings, and under the hereditary bonds 

 of the cell-habit. 



Irritability. This term is applied to that quality of living sub- 

 stance manifested when, by contraction or chemical change, it 

 responds to stimulation; and when the response is apparently 

 altogether disproportionate to the stimulating cause. Starch- 

 making under the influence of sunlight is not classified as a 

 form of irritability, since it is a reaction not particularly differ- 

 ent, in degree, from the reaction that goes on when a photo- 

 graphic plate is exposed to the chemical rays of the sun. There 

 is here no apparent disproportion between cause and effect and 

 the reaction does not essentially differ from similar changes that 

 might be initiated in altogether lifeless substance. But when 

 a nasturtium vine places its leaves perpendicular to the rays 

 of the sun, accomplishing this by a very complicated series 

 of changes in the shapes of cells and in the living substance 

 itself of the leaf-stems, there is a reaction out of proportion to 

 the immediate ca.use and possible only since a mechanism exists, 

 and this mechanism has been put in motion by the sunlight. 

 Irritable responses, as made by the plants and animals of to-day, 

 indicate not only mechanism capable of responding, but con- 

 note also an inheritance through the ages in view of which 

 the mechanism is what it is. As in all other manifestations of 



33 



