CHAPTER XVI. 



RESISTANCE OP PLANTS TO UNTOWARD INFLUENCES. 



1226. CLAUDE BERNARD has shown that life presents itself 

 under three forms : (1) latent, dormant, or inactive, illustrated 

 by the seed ; (2) variable, or oscillating, exemplified by the plant 

 during periods of apparent rest, when its activities are nearly 

 suspended, but when, in fact, some chemical changes are going 

 on, though very slight in degree ; (3) active, or free, exhibited 

 by a plant in full vigor. 



It has been repeatedly pointed out in previous chapters that 

 during their resting periods seeds and other parts can be sub- 

 jected to the action of influences which would destroy the life 

 of plants in full activity. 1 



1227. Inquiry as to the kind and amount of injury caused to 

 active plants by hurtful agents must deal with the influence of 

 extremes of temperature, too intense light, improper food, poi- 

 sons, and mechanical agents. Many of these injurious influences 

 and their effects upon special parts of the plant have already 

 been alluded to in previous chapters ; but it is proper to con- 

 sider them now with regard to the whole organism. 



1228. Effects of too high temperature upon the plant. Here, 

 as in most other cases, there is wide diversity among plants, 

 depending upon their constitutional peculiarities ; thus, plants of 

 the tropics not only demand higher temperatures than those of 



1 For some account of various recent views in regard to the nature of life, 

 the student is referred to the following works : Herbert Spencer, Principles of 

 Biology, 1870; Claude Bernard, Lecons sur les Phenomenes dela Vie communs 

 aux Animaux et aux Vegetaux, 1879; and Nageli's recent treatises. 



For an interesting account of the reactions of living matter to very dilute 

 solutions of certain substances which are poisonous when used in greater 

 strength, see Loew and Bokorny. These investigators use a dilute alkaline 

 solution of argentic nitrate in the discrimination between living and dead 

 protoplasm ; upon application of the reagent the former turns black, the latter 

 remains uncolored. The solution is made by mixing 1 cc. of a one per cent 

 solution of the nitrate in distilled water with an equal amount of a solution 

 containing 13 parts of potassic hydrate solution, 10 parts of ammonia, and 

 77 parts of distilled water (Pfluger's Archiv. xxv., 1881, p. 150). 



