112 CLASSIFICATION OF THE MICRO-ORGANISMS. 



spores (O'Oll mm. in diameter) sprout readily on the skin of 

 the abdomen of healthy flies, send out a germinating tube 

 which penetrates under the skin and there forms, by budding, 

 short round cells, which become detached and spread in the 

 blood (the germinating tube possesses a very sensitive mem- 

 brane which is at once dissolved in water, but not in salt 

 solution). These cells ultimately develop into tube-like hyphae, 

 of which one end projects through the skin of the posterior 

 part of the body, in the form of a club-like basidium. The 

 upper end of the basidium then forms spores by the production 

 of a pocket, into which plasma flows; this pocket, the future 

 spore grows, and finally becomes cut off by segmentation 

 from the basidium. Large vacuoles are then formed in the 

 latter, it takes up more and more moisture, and becomes 

 swollen; finally it bursts, and the contents spouting out, hurl 

 away the spore with considerable force. The empty tube 

 shrinks, and in its place a new one appears, in which the 

 same process is repeated. In this way the dusty layer of 

 spores around the flies is formed; the round spores (fig. 3) are 

 surrounded by a layer of plasma, which favours their adhesion 

 to the body of other flies. To this group also belong Empusa 

 radicans, observed in the caterpillars of the common white 

 butterfly, and Farichium megaspermum in the caterpillars 

 of Noctua segetum (Winter -saateule). 



Fungus of 3. Peronosporeae (order : Phycomycetes). Parasites 



on living plants ; the mycelium present between the 

 cells sometimes sends haustoria into their interior. All 

 these fungi can fructify asexually, fruit hyphae projecting 

 from the surface of the plant, and giving rise to conidia by 

 segmentation. The ripe conidia can at once germinate, 

 either by the direct formation of a germinating tube, or 

 by division of their contents into a number of portions, 

 which grow into swarming spores ; these ultimately 

 sprout like the other spores. Many peronosporeae, 

 however, possess in addition a sexual fructification. 



Species Peronospora ; about 40 varieties, attacking a num- 

 ber of phanerogams; mostly on green parts, the conidia 

 bearers being situated by preference on the under surface of 

 the leaves. The parts attacked become prematurely yellow 

 or brown, and die ; the conidia bearers form a fine, grey, 

 mould-like covering on the surface. Peronospora infestans : 

 fungus of the potato dis-ease ; mycelial tubes O005 mm. thick, 

 without haustoria; conidia bearers with 1 5 branches, thin 

 towards the top, with ellipsoid or egg-shaped conidia (fig. 4). 



This fungus has been known in Germany since 1830, and 

 was very destructive between 1845 and 1850; since then it 



