320 Transactions. 



The Tropic Movements of Phytophthora Mycelium. 



A peculiar feature of the biology of the mycelium of Phy- 

 tophthora infestans is that, after primary infection has taken 

 place on the leaf or stem, it is markedly positively geotropic. On 

 the other hand, as has been mentioned previously, the mycelium 

 developing from the dormant portion becomes negatively geo- 

 tropic. For my part, I attribute this not to the action of gravity, 

 but to the result of chemotactic stimulus, and that the mycelium 

 in all cases follows the direction in which food materials are 

 stored. This would account for the apparently contradictory 

 influence which gravity has up to the present been considered 

 to produce. 



An important point, and one which appears to have been 

 lost sight of, is the fact that in tomatoes attacked by Phytoph- 

 thora the direction of the mycelium is in general negatively 

 geotropic, or, as I take it, the mycelium is chemotactically 

 attracted by the food material stored in the fruit ; whereas in the 

 potato the mycelium is positively geotropic, being attracted 

 downwards by the chemotaclic stimulus of the tubers. 



Art. XXVII. — Note on the Gabbro of the Dun Mountain. 



By Dr. P. Marshall. 



Communicated by Mr. R. Speight. 



[Read befon the Philosophical Institutt of Canterbury, Wh December, 1907.] 



Captain Hutton first called attention to this rock,* and cor- 

 rectly described some of its peculiarities. Its coarse structure 

 and its simple composition (for it contains only two minerals) 

 were both noted. The specimen was given to him by Sir J. von 

 Haast, and the held relations of the rocks were unknown. 



The two minerals were called by Hutton anthophvllite and 

 saussurite, which he supposed to be derived from anorthite or 

 labradorite. In a later paperf he again classed the rock as 

 saussurite-gabbro, but classed the ferro-magnesian mineral as 

 diallage, enstatite, and hornblende in different portions. 



A geological report of the district by E. H. DavisJ does not 

 refer to gabbro rocks specifically, though it is probably in- 



•Trans. N.Z. Inst., vol. six, p. 412. 



f Journ. Roy. Sue. X.S.W.. vol. xxiii. p. 164. 



J Geological Reports, 1870-71, p. 103. 



