2 i-TNC-OID P?-:&TS OF CULTIVATED PLANTS. 



conidium is differentiated. This process goes on until a chain of conidia 

 is produced from the original branch, the apical conidium being the 

 oldest, and hence the first to separate itself from its companions, and so 

 the rest fall away in succession until they form a thin stratum of conidia 

 on the surface of the mycelium, in readiness to be transferred by wind or 

 rain to other and healthy leaves (PI. III. fig. 54ft). Upon reaching its 

 new location the conidium germinates by the production of a tube near 

 its extremity, and this germ-tube is the initial stage of a new mycelium. 

 This is the asexual reproduction, by conidia, of the oidium condition of 

 the Erysiphei, of which the ordinary European vine mildew and the 

 Australian Erysiphe viticola are examples. Later in the season the 

 threads of the mycelium produce a more complex form of fruit. A 

 globose receptacle, of a yellowish colour at first, is to be seen here arid 

 there upon the white mycelium. It seldom exceeds a small pin's head 

 in size, and ultimately becomes brown or black. The outer membrane, 

 or perithecium, remains attached, and is soon surrounded with more or 

 less distinct radiating flexuous threads or appendages, which vary accord- 

 ing to the genera (PL III. fig. 54ft). Internally the perithecium encloses 

 one, two, or more hyaline pear-shaped sacs, or asci, which contain the 

 sporidia. When mature the perithecia split irregularly, and the asci, 

 with their sporidia, are ejected. Each sporidium is elliptical, hyaline, 

 and capable of germination, the germ threads becoming a new mycelium. 

 This is the ascigerous and probably sexual reproduction. 



The whole career of these epiphytal parasites is therefore external 

 and superficial, and, if they can be destroyed by powdering or spraying, 

 the leaves may recover their vigour ; but if not, by the destruction of 

 the conidia or sporidia, or by their germination being prevented, the 

 disease is held in check, and its extension to other leaves or other 

 plants rendered impossible. The cultivator who possesses sufficient 

 elementary knowledge of the fungi to determine whether the pests he 

 has to deal with are of this nature is already in possession of the power 

 to treat them effectually. Even the very crude method of picking off the 

 diseased leaves and burning them will limit the area of infection. 



More important, and more destructive, are the endophytal parasites, 

 which originate within the tissues of the host-plants, and only manifest 

 themselves externally, when it is too late to save the plants. The " rot 

 moulds " are of this kind, such as the Potato mildew, American Vine 

 disease, Tobacco mildew, and many other devastating pests. They are 

 called " rot moulds " because of the rotting of the leaves and stems sub- 

 sequent to their attacks. Their scientific designation is PeronosporacecB, 

 and they have the habit and appearance of white moulds, but are para- 

 sitic on living plants. Here, again, it is of the utmost importance to 

 know something of their life-history, and methods of reproduction, before 

 they can be combated with success. The mature mould, when it appears 

 on the surface of a diseased plant, produces a profusion of spores, or 

 conidia. Each conidium is an elliptical colourless body, having a thin 

 outer coating of membrane with fluid contents. These contents soon 

 become granular, and at length collect at three or four centres, which 

 condense and then become distinctly separated from each other by the 

 growth of a special envelope. Ultimately the membrane of the mother 



