^continually permit the water which is in excess of that re- 

 - quired to maintain the cell in a state of turgidity to escape. 

 In terrestrial plants this excess water passes off in the form 

 of water- vapour and the phenomenon is termed transpiration. 

 Room is thus made in the cell for more water and a con- 

 tinual fresh supply of water and salts is provided for. 



86. Not Only solids, but also Manufacture 



gases, in solution are able to enter the cell by osmosis andp o j[ gan 

 oxygen and carbon dioxide, which are dissolved in the water Material, 

 surrounding it, are thus able to penetrate into the algal 

 cell. Now, provided that the temperature is suitable and 

 there is sufficient light, the protoplasm is able, by the help 

 -of the green chlorophyll, to decompose the water and car- 

 bon dioxide contained in the cell and to build up from 

 them an organic carbohydrate which is usually starch, oxygen 

 being given off. This is merely a preliminary process consist- 

 ing in the building up of a food material which in this case 

 precedes, and is quite independent of, nutrition proper, just 

 as the collection, or cooking, of food may precede actual 

 feeding. Some plants indeed exist which, as we shall see 

 later, can dispense altogether with this preliminary process, 

 although they are composed of living protoplasm which 

 requires feeding just as does that of green plants. Such plants 

 possess no chlorophyll, but by various means they are able to 

 obtain their organic food materials ready-made for them by 

 -green plants. 



87. Living protoplasm, as has Respiration 



been already noticed, respires, and plants, like animals, absorb Metabolism. 

 oxygen and give off carbon dioxide. During this process 



some of the substance of the protoplasm itself appears to be 

 destroyed and broken down into simple compounds, energy 

 being evolved which is in great part dissipated in the 

 form of heat. If therefore the protoplasm is to be main- 

 tained alive and enabled to grow, there must be a continual 

 supply of energy available by means of which the very com- 

 plex protoplasm may be again built up from simpler sub- 

 stances, and further in order that the protoplasm should 

 actually increase in bulk and grow, it is obvious that not 

 only must those molecules be replaced which have been des- 

 troyed but additional molecules must be constructed. This 

 constant source of fresh energy is, in green plants, supplied by 

 the starch which is being continually manufactured in the 

 orophyll corpuscles, and a considerable proportion of 

 which is a#ain being continually broken down into carbon 



