256 - Multicellular Plants 



in that, during photosynthesis, the leaves 

 produce more oxygen than they use in oxida- 

 tive metabolism. Likewise leaves in daylight 

 consume more carbon dioxide than they 

 form. Consequently the gas exchange of 

 photosynthesis overrides that of respiration 

 in the leaves of the plant during the daylight 

 hours. However, the stem and root, and the 

 leaves at night, must obtain oxygen from the 

 outside, and must get rid of carbon dioxide; 

 and this respiratory gas exchange takes place 

 through air spaces in the loosely packed 

 parenchyma tissues (Fig. 13-21). These air 

 spaces form a continuous system throughout 

 the plant, and in the leaves and stem, the 

 air spaces communicate with the outside 



Fig. 13-21. Diagram of the aeration system of a vas- 

 cular plant. The white lines represent intercellular 

 air spaces, opening to the outside through stomata. 



atmosphere through stomata. Roots have no 

 stomata, but the respiratory gases are ex- 

 changed by diffusion between the root tis- 

 sues and the soil water. Well-aerated soil 

 contains 20 to 40 percent (by volume) of air, 

 and this air maintains equilibrium with the 

 soil water, which clings to the loosely packed 

 soil particles. Closely packed soils — or 

 swampy soils in which the air spaces are 

 filled with water — are ill suited to most 

 plants, because these conditions tend to 

 asphyxiate the roots, eventually killing the 

 whole plant. Likewise a heaping of soil 

 around the lower parts of the stem may cause 

 asphyxiation. Each part of the plant must 

 breathe for itself, and the movements of the 

 respiratory gases depend entirely upon diffu- 

 sion. However, the oxygen requirements ol 

 plants are compartively modest, and each 

 plant, as a whole, produces more oxygen than 

 it uses. 



EXCRETION 



Aside from oxygen, the metabolic end 

 products of plants are chiefly water and car- 

 bon dioxide — the very substances that serve 

 as raw materials for photosynthesis. To a 

 large extent the plant utilizes its own pro- 

 ducts of combustion; and there is relatively 

 little excretion. The small amount of meta- 

 bolically formed water may remain in the 

 plant until it is utilized, or it may be passed 

 off by osmosis or transpiration. 



In plants, it is difficult to define the meta- 

 bolic wastes and to differentiate between ex- 

 cretion and secretion. Many flowers and 

 certain leaves give off sweet and fragrant 

 fluids in the form of nectar, which may be 

 partially excretory in nature. Some plant 

 cells tend to accumulate organic acids, espe- 

 cially oxalic acid, which may precipitate in 

 the vacuoles as crystals of the calcium salt. 

 Such crystals usually are formed in the leaves 

 or bark, where disposal occurs — by the shed- 

 ding of the leaves or the wearing of the 

 bark. 



