FOMES APPLANATUS 143 



great length of the spore-discharge period of Forties applanatus 

 is due, in part at least, to the fact that the hymenial tubes grow 

 continuously in length for several months and thus continuously 

 develop entirely new stretches of hymenium on their interior. 

 There is nothing in the growth of the gills of Agaricineae to corre- 

 spond to the slow elongation of the tubes of Fames applanatus. 

 Hence it is that the spore-discharge period of the Common Mush- 

 room, of Pleurotus ulmarius, etc., has a length of only about six to 

 eighteen days, whereas that of Fomes applanatus has a length of 

 six months. 



The Progressive Exhaustion of the Hymenial Tubes. It is not 

 unlikely that, toward the end of the annual spore-fall period of 

 Fomes applanatus, the lower part of the hymenium in each tube, 

 i.e. the part most recently developed, is liberating spores most 

 rapidly, whilst the upper older part is liberating spores either very 

 slowly or not at all owing to the exhaustion of most of its basidia. 

 If this surmise is correct, it would accord with the fact that, 

 notwithstanding the progressive increase in the total area of the 

 hymenium of a fruit-body, the diurnal spore-deposits, as observed 

 by White, were remarkably uniform throughout the spore-fall 

 period. It would also accord with what we know concerning the 

 gradual exhaustion of the hymenia of Panaeolus campanulatus, 

 Stropharia semiglobata, Psalliota campestris, etc. 1 I suspect, there- 

 fore, that the hymenium of each tube of Fomes applanatus exhausts 

 itself progressively from above downwards and that the rate of 

 its exhaustion is correlated, to some extent at least, with the rate 

 of the elongation of the tube in which it has developed. 



I have attempted to obtain evidence of the progressive exhaus- 

 tion of the hymenial tubes from above downwards, but so far 

 without success. On September 3, 1921, from the Kew fruit-body 

 already described, a piece of the most recently organised annual 

 tube-layer was obtained and found in one place to be 25 mm. 

 (1 inch) deep. From a block of hymenial tubes having this depth, 

 transverse sections were cut at intervals from below upwards and 

 examined with the low power of the microscope. It was ascer- 

 tained that spores were still being produced and liberated in certain 

 1 Vide infra, Chapters X, XI, XII, and XIII. 



