i28 A Textbook of theoretical botany 



these species occur on distinct host plants and differ from one another only in 

 minor particulars of the size of the spores. Recent investigations have 

 tended to split up certain of the older species. Thus the well-known form 

 Peronospora parasitica, which is widely distributed on Cruciferous plants, 

 has been split up into a number of separate species. One of the commonest 

 forms of this is Peronospora cheiranthi, which occurs on Wallflowers (Fig. 

 220). 



It lives as an obligate parasite within the tissues of its host plant and 

 spreads by means of a branched mycelium, which consists of hyphae that 

 ramify between the cells of the host. These hyphae do not penetrate into the 

 host cells but only pass between them. They give off, however, lateral 

 branches which enter the cells of the host and inside form branched or 

 swollen structures which are called haustoria (Fig. 221). These secrete 

 enzymes, by the aid of which the protoplasm of the host cells is broken down 

 and absorbed by the Fungus. Externally the Fungus causes a soft, white, 

 downy appearance on the leaves of the host plant, often associated with 

 twisting and stunting of the affected area. 



Peronospora parasitica is commonly found on Capsella bursa-pastoris (Shep- 

 herd's Purse), frequently associated with Cystopus candidus (White Rust) 

 (see p. 235). Other species are known to attack many crops of economic 

 importance, such as beet, maize, tobacco and turnips. A Fungus which, 

 like Peronospora, only attacks a specific host is said to exhibit selective 

 parasitism. 



Asexual Reproduction 



Peronospora reproduces both asexually and sexually. The asexual repro- 

 duction shows an advance on the condition found in Pythiiim, in that the 

 reproductive bodies are specially adapted for aerial distribution. The organs 

 are termed conidiophores, on which are borne specialized conidiospores 

 (Fig. 222). The conidiophores arise from the intercellular mycelium, emerge 

 through the pores of the stomata, and become repeatedly and dichotomously 

 branched. They may be to some extent septate. To the end of each of the 

 branches there passes a single nucleus ; a swelling occurs, and into this the 

 nucleus passes. The swelling increases until a spherical or oval spore is 

 formed, attached to the conidiophore by a short thin stalk or sterigma, 

 which is water soluble, a fact which facilitates distribution of the spores in 

 damp weather. 



Only a very small proportion of the conidiospores produced by the Fungus 

 ever come to rest on a suitable substratum, i.e., the leaf of another plant of 

 the same species of host, under environmental conditions suitable for 

 germination. The Fungus must therefore produce a very large number of 

 spores on the chance that a few will find conditions where germination is 

 possible. The presence of a water-soluble sterigma tends to ensure that 

 spores shall only be shed in weather when the humidity is sufficiently high to 

 render germination possible. 



