too 



BOTANY OF THE LIVING PLANT 



Under the normal day-conditions of a temperate climate the light 

 and temperature fall within the limits of intensity necessary for Photo- 

 Synthesis to proceed, while the Transpiration-Stream supplies the salts 

 required, including those of Potassium and Iron. These necessary 



conditions being met, the construc- 

 tive process consists in the absorp- 

 tion of carbon dioxide, fixation of 

 the carbon, release of free oxygen, 

 and the appearance in the active cells 

 of material formed from the carbon 

 which has been fixed. Commonly, 

 though not always, it is starch 

 that makes its appearance, and it 

 is located in the chlorophyll cor- 

 puscles themselves. Starch may 

 then be recognised as the first 

 visible product of Photo-Synthesis 

 (Fig. 72). 



Naturally the demonstration of 

 such a gaseous interchange as 

 that involved in Photo-Synthesis 

 cannot be given so readily in 

 sub-aerial as in submerged plants, 

 and the latter are usually used for 

 this purpose. If a single shoot 

 of the common Canadian Water 

 Weed (Elodea) be fixed below the 

 level of water in a glass jar, with 

 the cut end of the stern pointing 

 upwards, and the whole be ex- 

 posed to the light, bubbles of gas 

 will be seen to be extruded in 

 succession from the cut surface, 

 and to rise through the water 

 (Fig. 73). These are bubbles of oxygen. Arrangements can easily 

 be made to collect the gas in bulk from a mass of specimens, and 

 to subject it to the usual tests. These bubbles of oxygen are a visible 

 bye-product of Photo- Synthesis, and they may be used as a measure of 

 the activity of the process. The bubbles are approximately of equal 

 size, and the number of them liberated in a given time may be 

 counted. In this way it may be shown that the activity is greater in 



FIG. 72. 



A. Chlorophyll granules in cells of the leaf of 

 Funaria, showing small starch-grains included in 

 them. B shows stages of division of the chloro 

 phyll granules. / shows the included starch- 

 grains after the granule has swollen in water. 

 (After Sachs.) 



