338 Apple ami Pear Seal) F^ng} 



water for a sufficiently long time having once initiated the first stages 

 of germination, subsequent periods of desiccation seem to help rather 

 than hinder the growth of the fungus. The appearance of the germ 

 tube in the two species is characteristic, for in V. inaequalis it is rather 

 a continuation of the spore, being formed by the more pointed end 

 growing out as the germ tube; whilst in the case of the V. pirina it 

 is sharply defined, arising as a lateral growth near to one end of the 

 spore. 



In a hanging drop, the germ tube continues to grow out as a long 

 thin colourless hypha, but on the leaf desiccation soon brings the germ 

 tube in contact with the cuticle and this immediately brings about the 

 formation of an appressorium, the stimulus of contact, shown by 

 Aderhold, causing the germ tube to swell up and at the same time to 

 be closely applied to the cuticle. Some substance is excreted round 

 it so that a flange-like structure is formed round the end of the germ 

 tube, fixing it firmly to the cuticle (see Fig. 1). The changes in shape 

 of the fungal hypha and the exact method by which the flange was 

 formed could not be definitely followed, but the latter seemed to arise 

 as an excretion from the fungal hyphae, no part of the cuticle being 

 involved. At first the flat portion of the flange, which is usually very 

 thin, especially at the edges is colourless ; but it soon becomes brown 

 and can readily be seen under the microscope. These "collars" or 

 flanges are much more prominent in some cases than in others, apparently 

 depending upon the state of health of the spore and the conditions of 

 growth of the fungus. The nucleus can usually be seen to be very 

 evident in the appressoria denoting probably great metabolic activity 

 in this region. The formation of the appressorium is not limited to 

 the germ tube, as the tip of any growing hypha may respond to the 

 stimulus, although there is no evidence that the mature mycelium is 

 capable of doing so. If after the formation of the appressorium, the 

 spore is still kept immersed in water, the appressorium begins to put 

 out a hypha from its upper surface, and later its appearance may 

 suggest that it was a lateral outgrowth from the mature hypha. The 

 germ tube generally forms only one appressorium, normally at a short 

 distance from the spore, but sometimes it branches and even develops 

 a short superficial mycelium producing five or six appressoria. 



Aderhold and many other workers have noted that the germ tubes 

 frequently swell up info the appressoria over the junctions of the 

 epidermal cells, especially where three or more cells meet. Very 

 careful examination of inoculations in superficial view has been made 



