MILDEWS AND RELATED FUNGI 1 55 



toria are confined to the epidermal cells in all of the genera of the 

 family except Phyllactinia, which forms special hj^jhal branches which 

 enter the stomata, penetrate the intercellular spaces of the leaves and 

 finally send haustoria into the cells of the loose parenchyma. With 

 the exception of these haustoria, the mycelium of the "powder\' 

 mildews" is entirely superficial. The conidial forms of the different 

 fungi of the family were classified formerly under the name of Oidium, 

 but with a more detailed knowledge of their life history, this name has 

 been relegated to the synonymy. The conidiospores, which are formed 

 in great numbers, are carried by the wind, or by snails in the case of 

 Erysiphe polygoni on plants of Aquilegia and are capable of immediate 

 germination on reaching the epidermis of a suitable host plant, the 

 germ-tube penetrating the outer wall of some epidermal cell. True 

 sexual reproduction has been discovered in some of the mildews b}- 

 R. A. Harper, thus verifying the earlier observations of de Bary. 

 Sphoerotheca Castagnei serves to illustrate the process. The oogonium 

 and antheridium, which are formed where two neighboring hyphae 

 approach, each contains a single nucleus. The cell wall between these 

 organs is dissolved at the time of fertilization and the male and female 

 nuclei unite and a fresh wall is laid down between the two organs. 

 Now the wall of the future perithecium begins to form by the develo]> 

 ment of a number of upright hjrphal branches around the oogonium, 

 forming a pseudo-parenchymatous tissue, while other branches later 

 absorbed grow into the interior of the developing perithecium, while 

 the outer wall cells become flattened and darker in color. The fol- 

 lowing growth takes place in Sphcerotheca, which develops only a 

 single ascus. The carpogonium elongates, divides and a ciirved row 

 of five or six cells is formed. The penultimate cell of this row contains 

 two large nuclei, while the other cells of the row have one nucleus each. 

 The young ascus develops from this penultimate cell in which the two 

 nuclei fuse followed by a rapid increase in size of the ascus, which presses 

 against the inner wall cells of the perithecium and absorbs them. 

 The nucleus of the ascus finally divides three times, producing the 

 nuclei of the eight ascospores, which subsequently are formed by free 

 cell formation. From the half-grown perithecium there arise apical, 

 equatorial or basal hyphae which grow out as the appendages, or 

 suffulcra, which in Phyllactinia are acicular and bulbous at the base 

 (Fig. 53), in Uncinula hooked at the apex and in Podosphara and Micro- 



