866 



SCIENCE. 



[N. S. Vol. XIX. No. 492. 



the size of the spores are so diverse in the (+) 

 and ( — •) strains that systematists generally 

 would feel justified in describing them as 

 separate species. 



In heterothallic species, strains have been 

 found which from their failure to react with 

 (+) and ( — ) strains of the same form have 

 been called ' neutral,' and a similar neutrality 

 may be induced by cultivation under adverse 

 conditions. A table under preparation to de- 

 termine the relative abundance aiid distribu- 

 tion in nature of the (-+-), ( — ) and neutral 

 strains of Bhizopus has so far shown that, 

 although neutral strains are not uncommon, 

 the majority of the cultures, obtained from 

 various localities abroad and in this country, 

 belong to either the (-|-) or the ( — ) strain. 



In all species of both homo- and heterothallic 

 groups in which the process of conjugation 

 has been carefully followed, the swollen por- 

 tions (progametes) from which the gametes 

 are cut off do not grow toward each other, as 

 currently believed, but arise as a result of the 

 stimulus of contact between more or less dif- 

 ferentiated hyphae (zygophores) and are from 

 the outset always normally adherent. 



In some species the zygophores have been 

 demonstrated to be mutually attractive 

 (zygotactic). 



In the heterogamic subdivision of the 

 homothallic group, a distinct and constant 

 differentiation exists between the zygophoric 

 hyphaa and the gametes derived from them, 

 but in the remaining homothallic forms and in 

 all heterothallic forms no such differentiation 

 is apparent. Thus, while in the heterothallic 

 species the sexual difference inheres in the 

 whole thallus of either strain, in the homo- 

 thallic forms it is confined to the conjugating 

 branches of a single thallus. 



A process of imperfect hybridization will 

 occur between unlike strains of different 

 heterothallic species in the same or even in 

 different genera, or between a homothallic form 

 and ioth strains of a heterothallic species, 

 and distinct white lines are produced in many 

 cases at the regions of hybridization. 



By taking advantage of this fact it has been 

 possible to group together in two opposite 

 series the strains of all the heterothallic forms 



under cultivation. When thus grouped, the 

 ( — ) or less luxuriant strains will fall in one 

 series, while the (+) or more luxuriant will 

 be included in the other. 



From the foregoing observations it may be 

 concluded: (a) That the formation of zygo- 

 spores is a sexual process; (h) that the 

 mycelium of a homothallic species is bisexual ; 

 (c) while the mycelium of a heterothallic 

 species is unisexual; (d) and further that in 

 the (+) and ( — ) series of the heterothallic 

 group are represented the two opposite sexes. 



The writer intends during the coming year 

 to continue his investigations on the subject 

 of sexuality in the lower fungi, and would be 

 greatly indebted to any mycologists who might 

 be willing to assist him by sending culture 

 material of any of the forms of the Mucorinefe 

 which may be found producing zygospores. 



Albert Francis Blakeslee. 

 Cryptogajiic Laboratory, 

 Harvard University. 



on the de^'elopment of palisade tissue and 



RESINOUS DEPOSITS IN LEAVES. 



In connection with the experimental investi- 

 gation of the causes of xerophily in bog plants, 

 new evidence as to the factors involved in the 

 development of palisade cells and resinous de- 

 posits has been obtained. It has been found 

 possible, in the case of Bumex Acetosella L., 

 to greatly modify its external appearance and 

 internal structure by growing it under various 

 ecological conditions. When grown in moist 

 conditions, with soil and air temperatures ap- 

 proximately the same, the leaves attain a rela- 

 tively large size and their tissues are exceed- 

 ingly loose. A poorly developed palisade of 

 one cell-layer and three layers of spongy par- 

 enchyma, beneath it, make up the mesophyll. 

 The epidermis is composed of large turgid thin- 

 walled cells, having a very delicate cuticle on 

 the outside. 



When grown on dry sand the leaves are 

 notably thickened, reduced in size and the 

 margins become revolute. The mesophyll is 

 very compact and consists of a palisade of two 

 to three cell-layers and a spongy tissue of two 

 cell-layers. The epidermal cells are small and 



