Minnesota Plant Life. 505 



the arched form of leaf-stalk depends upon the action of pres- 

 sure, owing to the weight of the leaf-blade. All the beautifully 

 accurate arrangements of strengthening beams and elastic tissues 

 in stems and leaves may be regarded as possible only through 

 irritability of the living cells, some of which, under pressure, 

 thickened their walls to withstand it ; while others, in a region 

 of flexions, adopted elasticity of structure. 



Summary statement. What has been brought forward upon 

 the subjects of plant nutrition, growth and irritability, suf- 

 fices merely to direct attention to a few of the paths along 

 which investigation has proved profitable. No attempt has 

 been made in so limited a space to present a complete outline 

 of plant-physiology. In cursive manner, the existence of a 

 living substance has been affirmed and it has been intimated 

 that the physiological activity of the plant is in fact the physi- 

 ological activity of living substance. For this reason, ad- 

 vanced research must be with aid of the microscope and all 

 the other appliances of the modern laboratory. The secret of 

 the plant lies very deep and is not to be wrested from it by 

 superficial examination or analysis. 



Self defense. Not only must the plant utilize the materials 

 and forces about it and adapt its structure and behavior to the 

 environment, but it is also often called upon to defend itself 

 against unfavorable conditions. Two of the common animal 

 types of defense are but sparingly employed by plants. These 

 are active opposition and night. Not being provided with 

 muscles, the plant can rarely run from threatening dangers, nor 

 can it often combat them by an active display of force. Among 

 the lower plants flight is of defensive value for diatoms and 

 other motile varieties. They swim away from undesirable local- 

 ities and seek those where they can best maintain their exist- 

 ence. Motile cells, such as spermatozoids and swimming 

 spores, may also protect themselves in this manner. Active 

 opposition is very rare, but is practiced, perhaps, by such a plant 

 as the Venus's fly-trap when it snaps at the insect that may 

 have approached it with some notion of feeding upon its 

 tissues. 



Most plants maintain a passive defense against unfavorable 

 or harmful influences. A great variety of such defenses are 



