340 HOW PLANTS LIVE AND WORK. 



the forms which one studies, the more one is forced to the conclu- 

 sion that protoplasm has essentially the same nature, whether it 

 belongs to an animal or a plant. All protoplasm possesses the 

 power of spontaneous movement, and the energy which enables it 

 to move is derived from the oxidation, the " burning," of some of 

 its substance; just as the movement of an engine depends upon 

 the oxidation or burning of the coal or other fuel which drives it. 

 Thus, all living protoplasm is constantly using up oxygen, and 

 forming carbon dioxide and other simple compounds. We have 

 seen that in daylight green plants possess in addition the power of 

 taking carbon dioxide to pieces again, and fixing the carbon. To 

 make up for the waste of substance which the life processes entail, 

 fresh living matter must constantly be formed, and this cycle is 

 incessantly going on, in the Oak tree, in the cow which chews the 

 cud beneath its shade, and in the invisible organisms which turn 

 the cow's milk sour. 



But there is an immense number of plants which are not green, 

 for they possess no chlorophyll, and are therefore incapable of 

 manufacturing carbonaceous food for themselves. For this reason 

 many such plants — called Fungi — are parasitic on other plants. 

 There is before me a Rose tree which, although it is apparently in 

 good health, is doomed. Some of its leaves seem to be drooping, 

 and on turning them over one sees that they are covered below with 

 an exceedingly fine, silky network. This network consists of the 

 filaments of a Fungus, which is gradually draining the Rose of its 

 life-blood. Nearly all the diseases of crops and of forest trees are 

 due to certain Fungi. The moulds which destroy food are Fungi, 

 and the dry-rot which ruins timber is caused by organisms of the 

 same class. It would, however, be wrong to suppose that all 

 Fungi are injurious. Many — Mushrooms, for example — are 

 extremely good to eat, and others perform a most useful work in 

 setting up decomposition, and thus removing waste matters which 

 would otherwise accumulate and become a great nuisance. The 

 smallest Fungi, the Bacteria, have been found of late years to be 

 responsible for most, if not all, the infective diseases fto which 

 man is subject, and the investigation of the properties and life 

 histories of these microscopic plants is consequently a matter of 

 the most vital importance. It is already possible to control to a 



