THE PHYSIOLOOY OF PLANTS. 83 



resumed at the expense of the reserve material. From that 

 in the stems and roots the huds develop ; from that in the 

 bulb, a stem, leaf or flower-cluster may be formed ; from that 

 in the tuber the buds (" eyes ") develop into branches ; from 

 that in the seed the embryo grows into a self-supporting 

 plant; from that in the spore a germinative tube or other 

 form of growth proceeds. 



17. Respiration of plants, as of animals, consists in the 

 absorption of atmospheric oxygen and the hberation of car- 

 bon dioxide. This cannot readily be detected during the 

 daytime, when photo-synthesis is carried on. Nevertheless, 

 respiration is constantly performed, oxidizing the products 

 resulting from photo-synthesis and producing other chemical 

 changes. " The whole life of the plant consists in complicated 

 movements of the molecules and atoms; and the forces neces- 

 sary for these movements are set free by respiration." (Sachs.) 

 In the absence of oxygen, the chemical changes connected with 

 growth, the movements of protoplasm and the power of mo- 

 tion in motile and irritable organs cease. The heat generated 

 by oxidation seldom causes a sensible increase in the temper- 

 ature. But in some cases, for example in a mass of germinating 

 seeds or a heap of unfolding flowers, it can be detected. 



Put some Sunflower seeds (after removal of the hulls), in an open vessel of 

 water and they will germinate. Kepeat the experiment hut use a bottle full 

 o{ boiled (then cooled) water, avoiding any air space below the tightly fitting 

 cork. Since no air (free oxygen) is present no germination will occur. In 

 a glass jar or wide-mouthed bottle place a mass of germinating peas. Into 

 the centre thrust the bulb of a thermometer (preferably graduated to frac- 

 tions of a degree). Contrast the reading of this thermometer with another 

 (previously compared) outside the vessel. It would be advantageous to 

 cover both with a large bell jar. 



18. The limits of temperature at which photo-synthesis 

 and growth may take place vary in different plants. Some 

 live in very low temperatures, for example, the Red-snow 

 plant. In polar waters myriads of Diatoms and some of the 

 sea-weeds (Algae) flourish. The common plants of our region 

 require a comparatively high degree of temperature for the 



