136 



KNOWLEDGE 



[July 1, 1891. 



Fig. I. — Transverse section of 

 leaf of tomato (semi-diagram- 

 inutic). The leaf is upside down. 

 The luyceliiim (m ;/) of Fhtitoph- 

 thora infesians is raiuifving 

 through the mesophyll tissue 

 (/«), and a gonidium bearing 

 branch has found exit at a stoma. 



this fungus, as the writer 

 witnessed here last year. 

 Other plants (such as 

 the henbane) fouiid in 

 this order, are also sub- 

 ject to its attack. The 

 natural position of the 

 leaf is reversed, so that 

 the under surface is 

 uppermost. A thin 

 stalk, which branches 

 in the external medium, 

 issues from a stoma. 

 Eamifj-ing between the 

 mesophyll (middle-leaf 

 tissue) cells are thread- 

 like bodies connected 

 with this branched stalk. 

 The latter, in fact, is 

 merely a jjrolongation 

 of these threads. They 

 are the hyplne of the 

 fungus myceHum" which 

 nourishes itself at the 

 expense of the nutrient 

 matter in the cells of the 

 plant, and induces the putrefactive action that may 

 ultimately result in the total decomposition of the whole 

 plant. This fungus is thus distinctly a parasite. The 

 mycelium penetrates the cells by means of small processes, 

 liiiustiirin, or suckers ; they seem to have a putrefactive 

 action. The blocking up of the stomata or transpiring 

 organs of the plant prevents free evaporation of water, and 

 so hastens the decay initiated by the haustoria. The 

 branching filament figured as proceeding from the stoma 

 IS only one of myriads that may be found on one plant. 

 It is divided by a number of transverse divisions, and on 

 the ends of some of the branches, as well as on the main 

 stem, oval bodies are represented. Reference to Figure II. 



will show these 

 more distinctly. 

 The manner of 

 growth of this 

 branched stalk 

 merits description. 

 On the end of the 

 stalk and of its 

 branches a swell- 

 ing appears, and 

 is cut off in each 

 case fi'om the stem 

 on which it arises 

 by a transverse 

 septum. The bod- 

 I ies so produced are 



reproductive cells 

 as we shall present- 

 ly see, and hence 

 are termed r/oHirf/rt. 

 A slight swelling 

 appears on the 

 stalks below each 

 gonidium, and 

 afterwards growth 

 in length takes 

 place, the result being that the gonidia are pushed 



^, 



\ 



"^ 



70 n 



cC i o b h 



Fig. II. — More highly magnified view 

 uf gonidium bearing shoot {ad iiaf.). 

 <i. A single gonidium (still more highly 

 magnified); //. the >ame germinaling. 



* For explanation of terms see " The Travels and Life-History 

 Fungus," KsowiEDGE, June, 1891. 



aside and stand almost at right angles to their pedicels. 

 These prolongation pedicels repeat what has already been 

 described, and so on. 



A warm, humid atmosphere is especially favourable to 

 the development of these organs. This can easily be shown 

 by transferring a leaf with a poorly developed gonidiophore 

 (gonidium-bearing branch) from a comparatively dry and 

 cool medium to a bell-jar standing on water and situated 

 in a warm room. A rich crop of gonidia is the result. 

 These can live for three weeks in an atmosphere not per- 

 fectly dry. They are capable of germination in the presence 

 of moisture. In germination the wall of the gonidium 

 ruptures, and the protoplasm, enclosed in a very dehcate 

 membrane, protrudes as a small tube, which evidently 

 secretes a ferment, for it can force its way through the 

 cuticle of the epidermal cells of the leaf and thus iind 

 entrance into its interior. Once there it ramifies amongst 

 the cells, absorbing their nourishment, and acting in every 

 way like the parent from which it arose. If it be remem- 

 bered that the gonidia may be present in countless numbers, 

 the quick spread of the disease is easily accounted for. 



In addition to this method of increase there is another 

 asexual one. It generally happens that when plenty of 

 moisture is present the protoplasm of the gonidia breaks up 

 into eight pieces, each of which becomes more or less 

 rounded otf from its neighbour. The gonidium is now 

 equivalent to a sporangium or spore-containing vessel, but 

 as its contents ha>e arisen asexually the term gonidangium 

 is more applicable. In germination each portion of proto- 

 plasm sallies forth from the vesicle as an exceedingly minute 

 body, furnished with two vibratile processes termed cilia, 

 which are merely prolongations of its substance. Such 

 bodies are called zoogonidia, on accoimt of their capability 

 of active movement. This they exhibit only in water or a 

 nutritive tluid. They seem like animals, as they move rapidly 

 about from place to place in the watery medium. Their 

 exit from the gonidangium is prepared for by the swellmg 

 of the gelatinous apex of that body, and its almost simul- 

 taneous disappearance. At the same time the zoogonidia 

 themselves absorb water, but their extrusion from the 

 vesicle is directly caused by the swelling of the clear 

 gelatinous inner wall of that body, and probably also of 

 the clear layer that surroimds each zoogonidium. These 

 bodies can live, and only in water, a very short time. 



Suppose a few potato-plants in a large field of potatoes 

 to be attacked by this pest, it is perfectly evident that the 

 production of myriad zoogonidia on these during a damp, 

 warm day will cause the rapid spread of the di.sease. For 

 the smallest particle of moisture m the air affords a sufiS- 

 cient quantity of water for these minute bodies to live in. 

 The passage of a dog or other animal through the field, 

 and the visits of birds and insects, facilitate the spread of 

 this dreadful pest. 



The zoogonidia, if they find a suitable resting-place, at 

 once germinate. This process, as a rule, takes place on 

 the surface of a leaf. The cilia disappear, and a short 

 tube is sent forth. Like that of the parent gonidium, it 

 can pierce the epidermal cell-wall, and thus enter the leaf- 

 substance. In the leaf it increases in length and branches 

 like the parent mycelium. The hypliie so formed produce 

 gonidia and zoogonidia in their turn, and so the reproduc- 

 tion of the fungus may take place more than once in the 

 course of a single summer. 



Not only does the fungus affect the leaf, but it may 

 enter the stem and even penetrate into the tubers. More- 

 over, in the young state the tubers are subject to the 

 invasion of the gonidia and zoogonidia, to the former 

 especially if they lie exposed on the surface of the soil. 

 The latter, in certain soils, and in the presence of mois- 



