480 



BOTANY 



Excretion 



During the complicated processes of metabolism, many substances 

 are given off, which are not used for building up the protoplasm, 

 but are excretions. Some of these are strictly waste-products, like 

 the crystals of calcium-oxalate, or the alcohol and lactic or butyric 

 acid found in fermenting substances. C0 2 and water are always 

 excreted during respiration, and in the process of photosynthesis 



large amounts of free 

 oxygen are liberated. 



Some of the excreted 

 substances remain in the 

 plant, and are often of 

 importance, biologically. 

 Thus the poisonous alka- 

 loids developed in many 

 plants (nicotine, atropine, 

 etc.), the ptomaines and 

 toxalbumins, etc., are pro- 

 tective, or in the latter 

 .case, by killing the host, 

 Bacteria are provided 

 with a food supply. 



Other excreted sub- 

 stances may be useful in 

 other ways. Thus the 

 excretions of wax, or 

 mineral substances like 



FIG. 454. Excretion of water. A, leaf of Nastur- 

 tium. Drops of water excreted from water-pores 

 at the apex of the larger leaf-veins. (After 

 NOLL.) Z?, water execreted from the sporangio- 

 phore of Pilobolus (enlarged) . 



silica and calcium-car- 

 bonate, in the epidermal 

 cells of many plants are 

 useful either in preventing evaporation or giving rigidity to the 

 tissues. The secretion of nectar in flowers, or upon other parts of 

 the plant, and of pigments, is usually associated with the attraction 

 of insects. 



All activity of a living organism involves the breaking down of 

 matter, converting it to simpler chemical compounds, with a corre- 

 sponding freeing of energy. In its usual form, respiration consists 

 in the absorption of free oxygen, which, combining with the hydro- 

 gen and carbon of the organic compounds, produces C0 2 and water, 

 with an evolution of heat. This physiological combustion is what 

 is meant by Respiration. The carbohydrates, which have locked up 

 in them the energy of the sun's rays, compose the fuel which is 



