Lecture VI. 47 



mystery : Is the combination of the raw materials effected and 

 the energy introduced externally to, and without the alteration of 

 the pigment of the chloroplast, just as we might suppose that a 

 piece of clockwork might not only be manufactured from the 

 raw materials but also wound up by an engine? or does the 

 absorbed solar energy first make the raw materials combine with 

 the pigments and afterwards separate off the finished product 

 with a portion of solar energy embodied in it ? Our present state 

 of knowledge cannot answer these questions definitely, but there 

 is experimental evidence to show that the latter is the more 

 probable hypothesis, and the chemical relationship of the four 

 pigments of the chloroplast makes it appear that temporary com- 

 bination with them of the carbon dioxide to be used in the 

 synthesis is probable. 



To conclude, the outstanding and fundamental fact must not 

 be lost sight of, viz. that the minute chloroplasts of plants are 

 nature's engines for building organic substance from inorganic, 

 and by means of them green plants not only supply themselves 

 with food material and energy, but are actually the purveyors of 

 food and of the energy of the sun to the whole of living nature. 

 For naturally the food and energy which they supply to herbivor- 

 ous animals is transmitted by these to the carnivora which devour 

 them. 



PRACTICAL WORK. 



Cut off several short pieces from the tops of shoots of Elodea canadensis. 

 Immerse these in rain-water containing carbon dioxide dissolved in it. Note 

 that bubbles are discharged from the cut ends of the branches when the 

 leaves are exposed to bright light. The stream of bubbles slows down and 

 ultimately stops when the light is cut off. When one of these shoots is put 

 into rain-water which has previously been boiled to remove the dissolved 

 carbon dioxide no bubbles are formed on exposure to light. 



Mount a detached leaf in water and cover. With the low power observe 

 the general features of the leaf and note the dark lines in the central parts of 

 the leaf. These are the air-passages between the cells. Estimate the di- 

 mensions of the cells with the Ghost-micrometer. 



With the high power make a careful examination of one of the leaf cells 

 which shows its structure plainly. Observe the colourless cell-wall. In an 

 optical section of the vertical walls you will see their thickness. Compare 

 the thickness of the wall with the width of the cell. Note the numerous 

 chloroplasts lying close to the wall. Can you make out any lying near to 

 horizontal walls ? With a narrow diaphragm you will be able to make out 

 the protoplasm like a film against the inner surface of the cell-wall. The 

 chloroplasts are embedded in this film. Very often a nucleus can be seen. 

 It appears as a colourless transparent body lying in the cytoplasm, as the rest 

 of the protoplasm is called, and it has a higher refractive index than the cyto- 

 plasm. Look for its nucleolus and membrane. Notice the size of the vacuole. 

 Study the chloroplasts and look for starch grains in their substance. Make 



