472 BOTANY 



over a litre of water by evaporation, and a large tree several hundred 

 times as much. 



The great force exercised by a transpiring plant can be readily 

 shown by cutting off a vigorous leafy branch and fitting it air-tight 

 into a glass tube filled with water (Fig. 452). If the lower end of 

 the tube be placed in a vessel containing mercury, the latter will 

 rise to a considerable height in the tube, by the force of suction 

 resulting from the active transpiration from the leaves of the shoot. 

 In making this experiment, as well as others of a similar kind, the 

 end of the branch should be cut off under water, as otherwise air 

 may be drawn into the cut tracheids, which prevents the entrance of 

 water, and in consequence the leaves wither, as they are unable to 

 replace the water lost by evaporation. 



PHOTOSYNTHESIS 



The most important of the nutritive processes in green plants is 

 the assimilation of C0 2 . This is accomplished by the chloroplasts, 

 which absorb and use the energy of the sun's rays. With the excep- 

 tion of a small number of Bacteria, this power of photosynthesis, as 

 it is called, is peculiar to plants with chlorophyll, which are, there- 

 fore, the primary sources of carbonaceous food for all organisms. 



As the supply of C0 2 is being constantly renewed in the atmos- 

 phere, plants do not ordinarily need to shift their position in the 

 quest for food, as animals do, and this may be the explanation of 

 their immobility. Where they are destitute of chlorophyll, they 

 must, like animals, depend upon other plants (or animals) for their 

 sustenance. 



A necessary condition for photosynthesis is a certain amount of 

 light, varying in intensity in different cases. Artificial light, espe- 

 cially electric light, is also efficient for photosynthesis, but ordinarily 

 photosynthesis is dependent upon sunlight. 



Function of Chlorophyll. The chloroplasts, or green chromato- 

 phores, are the organs directly concerned in the process of photo- 

 synthesis. The importance of the chlorophyll in the chloroplast 

 consists apparently in its absorbing certain light-rays, whose energy 

 is thereby rendered available to the plant. Faint traces of the 

 assimilation of C0 2 have been detected in etiolated chromatophores. 



Accessory Pigments. Where the chlorophyll is accompanied by 

 other pigments, the latter are supposed to influence the rays of light 

 absorbed by the chromatophores. Thus in the Brown Algae, which 

 as a rule are floating plants, and are often completely exposed at low 

 tide, it may be that the brown pigment (phycophaein) serves as a 

 screen which cuts off excessive light, while the red pigment (phyco- 

 erythrin), found in the Rhodophycese, which as a rule live in deeper 



