284 PAEASITISM IN PLANTS AND ANIMALS. 



various grasses, and every one must have remarked tliat where 

 those pretty but noxious weeds abound the grass crop is usually 

 poor and stunted. 



Time will not allow of more than a very brief reference to the 

 numerous parasitic organisms known as bacteria, and which are 

 so fruitful a source of disease. Recent additions to our know- 

 ledge of these minute "fission-fungi" have shown not only the 

 truly vegetable nature of the organisms themselves, but have 

 placed beyond doubt the fact that they are the actual causes 

 of many of the most serious diseases to which man and other 

 animals are subject. I shall show you on the screen photographs 

 of some of the most familiar of these bacilli — those found in such 

 diseases as tetanus (lock-jaw), Asiatic cholera, and consumption. 

 Pray understand that these bodies are of the most extremely 

 minute dimensions, quite invisible except by high powers of the 

 microscope and measurable only by thousands of an inch. They 

 grow with the most astounding rapidity, by a process of repeated 

 division or fission, and after a time this process is supplemented 

 by the formation of spores, or specialised reproductive bodies — 

 comparable to the seeds of ordinary plants. Somewhat larger 

 examples of parasitic forms amongst fungi are the growths {Em- 

 pusa) which attack the common house-fly at the end of autumn, 

 forming white, mildewy patches upon its body, — the fungus of 

 salmon and potato-disease {Saprolegnia and PhytopMhora), and 

 that which attacks the human skin and hair, producing ring- 

 worm. In this disease the hair is seen to be filled with the 

 mycelium or thread-like filaments of the fungus, producing 

 abundantly spores or reproductive bodies. The hair thus be- 

 comes quite disorganised and breaks off short, leaving bald 

 patches. 



Among the most interesting episodes of animal life are the 

 histories of the development and migrations of internal parasites 

 such as the numerous worms and flukes whicli are found inha- 

 biting the stomach, intestines, respiratory organs, and various 

 tissues of the higher animals. Only in a few cases has the entire 

 life-history of these creatures been fully made out, but wherever 

 this has been done we find that the parasite, before attaining 



