116 



Mr. E. S. Salmon. Cultural Experiments with [Dec. 2, 



is much increased. On the other hand, if they are made to slope away 

 from each other the glider becomes unstable. 



9. Two square planes of equal size placed one behind the other at a 

 small angle are less stable in the examples considered than a square 

 equal in size to one of the two, but more stable than a single square 

 whose side is equal to the total length of the glider formed by 

 the pair. 



10. A pair of unequal squares of which the smaller forms a rudder 

 are more stable, in the examples considered, when gliding with the 

 rudder behind than with the rudder in front. 



11. In general, the stability is increased by making the moment of 

 inertia as small as possible. 



" Cultural Experiments with ' Biologic Forms ' of the Erysvphaceoe" 

 By Ernest S. Salmon, F.L.S. Communicated by Professor 

 H. Marshall Ward, F.B.S. Keceived December 2, 1903 — 

 Read February 4, 1904. 



(Abstract.) 



In the introductory remarks the author points out that through 

 specialisation of parasitism " biologic forms " have been evolved in the 

 Erysiphacece which, both in their conidial (asexual) stage and ascigerous 

 (sexual) stage, show specialised and restricted powers of infection. 

 .The powers of infection, characteristic of each "biologic form," are 

 under normal conditions sharply defined and fixed, and hitherto the 

 result of the experiments of numerous investigators — both in regard 

 to the present group of fungi and to the Uredinew, where the same 

 specialisation of parasitism occurs — has been the accumulation of 

 evidence tending to emphasise the immutability of " biologic forms." 



The second part of the paper gives the result of cultural experi- 

 ments with " biologic forms " of Erysiphe Graminis DC, carried out 

 during the past summer in the Cambridge University Botanical 

 Laboratory. It has been found that under certain methods of culture, 

 in which the vitality of the host-leaf is interfered with, the restricted powers of 

 infection, characteristic of " biologic forms" break down. 



In the first method of culture adopted, the leaf, which was either 

 attached to a growing plant, or removed and placed in a damp 

 chamber, was injured by the removal of a minute piece of leaf -tissue. 

 In this operation the epidermal cells on one surface, and all or most 

 of the mesophyll tissue, were removed at the cut place, but the 

 epidermal cells on the other surface (opposite the cut) were left un- 

 injured. Conidia were sown on the cuticular surface of the uninjured 

 epidermal cells over the cut. In a few experiments the conidia were 



