K.— BOTANY. 187 



Already among these fungi the development of the contents of the 

 oogonium without fertilisation has become common, and information is 

 beginning to accumulate as to the physiological conditions which determine 

 the appearance of male or female organs or of both. Thus Klebs was 

 able to maintain Saprolegnia ferox' in a vegetative condition so long as 

 fresh, unaltered nutriment was provided, but sporangia appeared on 

 transfer of the mycelium to pure water, and gametangia in the presence 

 of staling products, especially when the food supply was sufficiently concen- 

 trated to prevent sporangial development. In nutrient solutions poor in 

 phosphates parthenogenetic oogonia were obtained, while both Saprolegnia 

 ferox and Achlya polyandra^ give rise, on protein substrata, to abundant 

 antheridia and oogonia in the presence of calcium phosphate, and to a 

 smaller number when provided with phosphates of sodium or potassium. 

 In Phytophthora erytkroseptica an increase in the proportion of available 

 carbohydrate has been found by Dr. Barnes to limit the formation of 

 gametangia, and it is well known that, in nature, the vegetative develop- 

 ment and sporangial activity of a number of species takes place on the 

 living host, whereas the sexual organs appear when the host is dead and 

 the fungus is growing as a saprophyte ; doubtless, under these conditions, 

 staling products tend to accumulate. 



In the great majority the mycelia are capable of bearing both male 

 and female organs, but Phytophthora Faberi^ and species of Dictyxichus^^ 

 have been shown to be dioecious. 



In the Mucorales sexual reproduction has long been known to take 

 place by the union of large, multinucleate gametangia. These may be 

 similar in form or recognisably male and female, they develop in contact, 

 the wall between them is dissolved, and their contents mingle without the 

 intervention of a conjugating tube. In many species the gametangia can 

 be obtained with ease, and their appearance is clearly associated with a 

 suitable provision of food and water. Thus in Sporodinia grandis^^ the 

 fertile hyphse are rich in glycogen and their formation is conditioned by 

 the presence of carbohydrates as well as by a saturated atmosphere. In 

 other members of the alliance, gametangia proved most uncertain in their 

 development, and it was not till 1904 that Blakeslee^^ was able to show that 

 they appeared only along the line of junction of two separate mycelia. 

 There was here a new conception of sexual differentiation ; since the 

 gametangia were similar both in size and behaviour, it was impossible 

 to describe one as male and the other as female ; yet they, and the mycelia 

 which bore them, clearly differed in an essential character, and Blakeslee 

 applied to them the arbitrary designations of (-f ) and (— ). In some 

 cases a difference of vegetative luxuriance distinguished the two strains, 

 in others a (+) or a (— ) strain could only be defined by its capacity to 

 produce zygospores with the other. It seemed evident that there existed, 

 in effect, both in the sexual organs and in the thalli which bore them, a 

 physiological differentiation of sex unaccompanied by morphological 

 distinction. To species possessing (+) and (— ) strains Blakeslee applied 

 the term heterothallw, using homothallic for those in which zygospores could 

 be obtained in single spore culture. 



In Mucor Mueedo^^ the power of conjugation may be inhibited by 

 unfavourable conditions, but so far nutritive or other factors have not 



