HARDWICKE'S SCIENCE-GOSSIP. 



251 



is aquatic in one stage of its existence, and in that 

 stage the resting spores are formed. 



Reference may here be made to the bodies found 

 germinating in the intercellular passages of spent 

 potatoes by Dr. Montagne (Artotrogus), and re- 

 ferred by Mr. Berkeley to the Sepedoniei. Ever 

 since Mr. Berkeley first saw these bodies he has 

 had an unswerving faith in the probability of their 

 being the secondary form of fruit of Peronospora 

 infestans, but unfortunately, as far as I know, no 

 one has ever found a specimen of Artotrogus since 

 Montagne. 



The question may, therefore, be naturally asked, — 

 How does Artotrogus agree with the presumed 

 resting spores here figured and described? And 

 has Mr. Berkeley been right or wrong in clinging 

 so tenaciously to his first idea? Fortunately for 

 the investigation of the potato disease (which can 

 never be cured till it is understood), Mr. Berkeley 

 has given in the Journal of the Royal Horticultural 

 Society the number of diameters his figures are 

 magnified to, and I have here engraved those figures 

 so as to correspond in scale with my own drawings, 

 which latter are sketched with a camera lucida. 

 It will be seen that they are the same with each 

 other both in size and habit, with the exception of 

 the processes on the mature spore of Artotrogus — 

 which processes may possibly be mere mycelial 

 threads, or due to the collapsing of the inflated 

 epispore. The reason why these resting spores have 

 evaded previous search is that no one has thought 

 of finding them amongst leaves which had been 

 macerated for a long period in water. There is, 

 however, nothing unreasonable in fruit being per- 

 fected in water or very damp places, as it is common 

 in the Saprolegniece and amongst Algse in general. 

 To sum up, there are four reasons why the bodies 

 here described belong to the old potato disease : — 



1. Because they are found associated with the 

 Peronospora and upon the potato plant itself. 



2. Because they agree in size and character with 

 the known resting spores of other species of Perono- 

 spora. 



3. Because some oilier moulds are aquatic in one 

 stage of their existence. 



4. Because they agree in size with Artotrogus. 

 Now that these drawiugs illustrative of the 



fungus which causes the potato murrain are repro- 

 duced in the following plates, it may be as well to 

 explain at once some of the terms used and the 

 nature and habit of the bodies hereafter referred to, 

 for such readers as may not be thoroughly acquainted 

 with the life history of the destructive parasitic 

 moulds to which the potato fungus belongs. Eor 

 that purpose reference must be made to fig. 159, 

 which shows (greatly enlarged) a transverse section 

 through the leaf of a potato plant ; the two great 

 bodies at a a represent two minute hairs on the 

 leaf, and at b b are seen the individual cells of 



which the leaf is constructed. When these hairs and 

 cells are compared with the fine thread at c, which 

 represents a branch of the potato fungus coming out 

 of a breathing pore of the leaf, it will be seen how 

 very minute the fungus is in comparison with the 

 dimensions of the leaf. This fine thread is no other 

 than a continuation of a thread of spawn or myce- 

 lium which lives inside aud at the expense of the 

 assimilated material of the leaf. When this thread 

 emerges into the air, as here shown, it speedily 

 ramifies in different directions, aud bears fruit at the 

 tips of the branches, as at d d; these fruits are 

 termed simple spores, or conidia, because from their 

 smallness they are dust-like. It is quite possible 

 they may be an early state of the vesicles which con- 

 tain the zoospores, as seen at e, e. However this 

 may be, they are commonly arrested in growth when 

 still small, aud they germinate in an exactly similar 

 manner with the zoospores themselves, and may be 

 considered somewhat analogous with seeds. The 

 potato fungus .has another method of reproducing 

 itself in the " swarm-spores " shown at e, p. These 

 are so called because, on the application of moisture 

 (as supplied by dew or rain, or when applied 

 artificially), the vesicles set free a swarm of from 

 six to fifteen or sixteen other bodies known as 

 «' zoospores," so named because they are furnished 

 with two lash-like tails, and are capable of moving 

 rapidly about like animalcules. This rapid move- 

 ment usually lasts for about half an hour, and (like 

 the dust-like conidia or " simple spores " before 

 mentioned) the swarm-spores generally enter the 

 breathing pores of the leaf, and there germinate. 

 So potent, however, are the contents of these bodies 

 when set free, that they are capable of at once cor- 

 roding, boring, and entering the epidermis of the 

 leaf, or even the stem or tuber itself. These 

 zoospores are best seen when within the vesicle f, 

 where they arise from a differentiation of the con- 

 tents, but when once set free (g) they are, from the 

 extreme rapidity of their movements, very difficult 

 to make out. In about half an hour they cease to 

 move, their lash-like tails (cilia) disappear, and 

 having burst at one end, a transparent tube is pro- 

 truded, which is a similar mycelium in every respect 

 with that produced by the simple spore, aud which 

 grows, branches, and fruits in a precisely similar 

 manner. 



Now the great difficulty which has beset botanists 

 for so many years has been to account for the winter 

 life of the potato fungus. Simple spores and zoo- 

 spores are lost in the production of the mycelium or 

 spawn, and this latter fine thread-like material 

 cannot of course survive the frosts and rains of 

 winter, but must utterly perish with the perished 

 leaves and haulm. 



A study of other species of Peronospora allied to 

 the one which produces the potato disease, reveals 

 the fact of a third mode of reproduction. Simple 



