CH. I] ASSIMILATION. 11 



certain patches or stripes which are yellowish-white in- 

 stead of being green, because they contain no chlorophyll. 

 If such a plant is placed in the dark the leaves will 

 after a time become starchless; if it is then exposed to 

 light, starch will appear, but only in the green parts where 

 chlorophyll is present. Moreover it is possible with the 

 help of the microscope to see that it is in the chloroplast 

 that the starch appears and disappears. This is 

 especially evident in Spirogyra, where the starch in the 

 form of minute granules is gathered round certain centres 

 in the chloroplasts which are known as pyrenoids. 



The fact that the green plant is a machine driven by 

 the energy of sunlight can be made evident to the eye by a 

 well-known observation. When a water-plant, such as the 

 common river- weed Elodea, is placed in a beaker of spring- 

 water and exposed to sunshine, streams of minute bubbles 

 are seen to issue from the cut stalks. If the beaker is 

 darkened the bubbles cease and the same thing happens if 

 the water is freed from C0 2 . The bubbles contain the 

 oxygen that is set free in the process of assimilation: it may 

 roughly be said that the plant seizes the carbon from the 

 C0 2 and lets the oxygen go. It is obvious therefore that 

 if there is no C0 2 in the water the production of oxygen 

 must cease, and the fact that the bubbling stops in the dark 

 shows that light is the power which drives the machine. 

 The stream of bubbles pouring from a water weed in 

 sunlight is, like the smoke coming from the chimney of a 

 cotton-mill, a sign of internal activity. The chimney 

 ceasing to smoke may mean either that there is a want of 

 cotton, a want of coal, or that the machinery is broken. 



