18 DISEASES OF TROPICAL PLANTS CH 



sugars and starches, namely, cane sugar (C 12 H 22 U ), 

 grape sugar (C 6 H 12 6 ), and starch (C 6 H 10 5 ), at the 

 same time setting free a part of the oxygen, which is 

 returned to the air and can be utilised by animals. A 

 part of these starches and sugars is utilised in the 

 construction of the cell walls of the plant ; part is 

 removed to other parts of the plant and either used for 

 growth or stored up for future use ; and, finally, part is 

 reconstructed in combination with the salts which were 

 taken up in solution in the water and formed into proteids 

 and fats. The sun being the source of all energy, the 

 greater part of the work must be done in the sunlight, 

 but the process is by no means clearly understood. 

 The transferring and storage of these food substances 

 in the fruits, tubers, roots, etc., for future use is accom- 

 plished largely during the night. 



The magnitude of the work done by plants cannot 

 be comprehended. The enormous number of plants 

 with their varied character, feeding mankind and all 

 forms of animal life, and either directly or indirectly 

 furnishing us with clothing and fuel and a great part 

 of our building material, are nothing more nor less than 

 the combination of these raw materials which have 

 been influenced in some mysterious manner by the 

 sun's energy. Although the leaves are the great 

 laboratories for the manufacture of foods they have 

 also been called the lungs of the plant, for it is through 

 them that the respiration or breathing takes place. In 

 this process, the plant gives off some carbon dioxide 

 and takes up oxygen, but the ratio between the two 

 is quite different in different plants, and can be only 

 briefly referred to at this time. The leaves also give 

 off water through the stomata, which are so regulated as 

 to control the transpiration. 



All plants that contain chlorophyll, whether the 

 simplest of the algae, the liverworts, mosses, ferns, or 

 the highest of the spermatophytes, are able to perform 

 these duties of taking the gases from air, water and 

 salts from the soil, and building them up into complex 



