498 Minnesota Plant Life. 



the living substance, irritable phenomena are related with past 

 time as well as with the present. Here, indeed, lies one of the 

 principal differences between a living thing and an insentient 

 mechanism composed of springs, wheels, pulleys, levers and 

 rods. One has a past ; the other is a creature of the present. 



Irritable behavior of living substance. While all manifes- 

 tations of irritability must be attributed to responses by the 

 living substance, it is convenient to examine first those responses 

 made by living substance, as such; and later to consider the irri- 

 table responses of cells and organs. A further distinction may be 

 made, for purposes of analysis, between induced and automatic 

 responses. Really, all responses are induced by stimuli ; but 

 if these stimuli originate within the living substance as a result 

 of subtle rearrangements of its component structure the re- 

 sponses may then be termed automatic, in contradistinction to 

 responses which are plainly induced by external stimuli. Thus, 

 the extraordinary marshalling into groups of the component 

 parts of a mass of living substance, before division of a cell 

 takes place, would be termed automatic ; the lashing of a swim- 

 ming thread, such as that of a spermatozoid, is also automatic ; 

 but the contraction of a portion of protoplasm when exposed 

 to a slight electric shock would be regarded not as automatic 

 but as induced. 



Examples of automatic irritable phenomena in living sub- 

 stance are very numerous. Here should be classified the stream- 

 ing movement, the lashing movement, the crawling movement, 

 and the spontaneous contractions of protoplasm. Here, too, 

 should be grouped those amazing evolutions by which the 

 nucleus of a cell divides itself into two, prior to cell division. 

 It is by no means a mere halving of the nucleus that then 

 takes place ; but a complex segregation of certain portions on 

 one side and certain portions on the other side of a neutral line, 

 the whole recalling military marchings and countermarchings, 

 or the working of some ingenious manufacturing contrivance, 

 such as the pressroom of a metropolitan newspaper. Automatic 

 movements are dependent upon suitable outward conditions, 

 as are also induced movements. Thus, too high or too low a 

 temperature, too strong or too weak illumination, or some 

 other condition of the environment, may prevent their appear- 



