42 BOTANY 



Nutrition of Protoplasm 



No less characteristic than its motility, is the ability of proto- 

 plasm to assimilate food. For this process the presence of water 

 is as essential as it is in movements. Dry protoplasm is incapable 

 of nutritive activity, as water is necessary both for the physical 

 and chemical processes connected with nutrition or metabolism. In 

 plants food can only be taken into the cells in solution, so that 

 water is a necessary vehicle for the transport of food elements ; and 

 finally the decomposition of water itself is the source of the hydro- 

 gen and part of the oxygen which enter into the carbohydrates 

 manufactured in the green cells under the influence of light. 



Through the activity of the protoplasm the food elements undergo 

 various changes until they form new elements for building up the 

 protoplasmic substance, which thus increases in amount, or grows. 

 All of the metabolic processes, however, are not constructive, and 

 there are formed also certain waste products. Some of the waste 

 products arise from the decomposition of the protoplasm, with an 

 evolution of energy. The most familiar of these destructive meta- 

 bolic processes is respiration, where the atmospheric oxygen acts 

 upon the carbonaceous protoplasmic structures, which are decom- 

 posed, yielding as waste products carbon-dioxide and water, and 

 evolving heat. 



Irritability 



Irritability, or response to external stimuli, is a universal attribute 

 of protoplasm. Light, heat, moisture, mechanical shocks, electricity, 

 and many chemical substances exercise marked influences upon the 

 protoplasm. 



Light. Protoplasm is often exceedingly sensitive to the action 

 of light, whose effects are especially noticeable upon the green cells 

 of plants. The movements of zoospores, and of the chloroplasts 

 within the cell, have already been alluded to. Here the importance 

 of the light-rays in the assimilation of carbon-dioxide is the reason 

 for the movements. The movements of free-swimming green cells, 

 like the zoospores of any Alga in which these are freely produced 

 e.g. Chaetophora, Ulva are most striking. If the Algae are placed 

 over night in a glass or porcelain dish, of which one side is more 

 strongly illuminated, the masses of motile cells will be found in the 

 morning collected on the lighted side, and visible to the naked eye 

 as a deep green line on the surface of the water. If a few of the 

 active spores are examined under the microscope, they will be found 

 to swim to the side of the slide toward the window. In these motile 

 green cells there is very often present a red pigment-spot, which is 

 associated in some way with the perception of light, and is compar- 



