HARDWICKE'S SCIENCE-GOSSIP. 



the characteristic swellinj^s on tlie branched threads 

 are shown, the stems bearing an abundance of 

 spores(which are analogous with seedsand reproduce 

 the parent) on their apices ; the tlircads, as in tlie 

 last figure, are seen emerging through a breathing- 

 pore on the lower surface (here inverted, better to 

 display the character of the fungus). 



It is easy to see from this figure the damaging 

 effect the fungus must have upon the plant : the 

 fungus stems protrude from its mouths, and prevent 

 the emission of perspiration ; the potato-plant thus 

 gets surcharged with moisture, which rots the 

 stems and leaves, whilst the mycelium preys upon 

 the tissues. 



rainy weather, or after heavy dews, and when 

 the leaves of potato-plants are all wet and blown 

 against each other by Ithe wind, a few zoospores, 

 originating from two or three infected plants, would 

 speedily contaminate a large field of potatoes : then, 

 when we remember the hundreds of thousands of 

 ripe ordinary spores blown about everywhere by the 

 wind, their rapid germination, and immediate re- 

 production of other ripe spores and new zoospores, 

 the rapid and fatal spread of the murrain remains 

 no longer a mystery. 



Fig. 155. Peronospnra infestans, spores and zoospores, 

 enlarged 400 diameters. 



When the mature spores (G, fig. 155) fall from 

 their apices, they readily germinate, as at H, H, by 

 rupturing their outer coat, and discharging their 

 contents : these contents immediately take the 

 form of confluent mycelial threads, and produce the 

 characteristic brown colour in the cellulose. The 

 spores in this figure are enlarged 400 diameters, 

 or to the same scale as fig. 153. In fig. 153, how- 

 ever, it must be remembered that the spores re- 

 presented are immature. In the perfect condition 

 of the potato fungus, certain privileged spores ac- 

 quire greater dimensions than others, as shown at 

 J, fig. 155 ; the contents of these privileged spores 

 become differentiated, and produce within them- 

 selves a number of distinct nucleated cells, wliich 

 at length are set free in the form of active zoo- 

 spores, each zoospore being furnished with two 

 thread-like processes (K), with which, when in 

 fluid, they are enabled to move rapidly about. 

 These bodies germinate exactly iu the same 

 way as the ordinary spores, by discharging their 

 contents through the ruptured outer coat (L), 

 and must play a very important part in the 

 economy of the plant, for it is manifest that 

 although they cannot move unless immersed 

 in fluid yet it can easily be imagined that during 



Fig. 156. Periinospiira in/esfiins, spore germinating, 

 enlarged 400 diameters. 



Fig, 156 shows a section through the stalk of a 

 potato-plant, with a single mature spore germinating 

 upon the surface, its mycelium penetrating the 

 epidermis (M) and cortical layer (N N). 



Now, not only is Peronospora infestans able to 

 reproduce itself from its spores and zoospores, but 

 amongst the mycelium iu the intercellular passages 

 of spent potatoes are found other bodies which 

 there grow and fructify. These bodies, discovered 

 by Dr. Payen, though referred to the Sepedoniei 

 by Montague (the order next in succession to Muce- 

 dines, to which latter order the genus Peronospora 

 belongs), are considered by Berkeley and others to 

 be probably a secondary form of fruit (oospores) 

 of the potato fungus itself. These bodies, named 

 by Montague Artotrogus hydnosporus, are shown in 



Fig. 157. Artotrogus hydnosporus, enlarged 400 diameters, 



fig. 157 magnified 400 diameters ; the young Arto- 

 trogus being shown at in its mother cell (with 

 threads), and at P free. 



