THE EXTENT OF THE HYMENIUM 31 



By the weighing method already described, the area of the gills, 

 including both sides, i.e. H, was determined to be 1942 cm. 2 

 approximately. 



Hence, Sp. Inc. = -"' - = 16 approximately, 



i.e. the fruit-body had sixteen times more hymenial surface than 

 it would have had if the underside of the pileus had not been 

 produced into gills. 



In the case of the Mushroom, the gills on one quarter of 

 the pileus were isolated one by one, and their outlines marked 

 out on paper. The figures were then cut out and weighed 

 against paper ruled into square millimetres. The area so deter- 

 mined was multiplied by four, and thus the whole surface area 

 of the gills obtained. 



From the above table it will be seen that the common field 

 Mushroom has the highest specific increase, namely 20. This is 

 not surprising, for field Mushrooms have deep gills closely packed 

 together (cf. Plate IV., Fig. 25). Russula citrina, on the other 

 hand, has much shallower gills of one length only, which are placed 

 at some distance apart. The specific increase in this species is 

 consequently very small: it is only 7, i.e. one-third of that of the 

 Mushroom. Again, it is clear that with pilei of equal diameters. 

 Hypholoma sublateritium has considerably more gill-surface than 

 either Amanita rabescens or ArmiUaria meUea. If we take the 

 specific increase of gill-surface as a test, it seems fair to conclude 

 that of the fungi investigated, the greatest morphological advance- 

 ment is exhibited by Psalliota campestris and Hypholoma sub- 

 lateritium, and the least by Russula citrine. 



In the Polyporese the formation of hymenial tubes often 

 leads to a considerable increase in the spore-bearing area. 

 The amount of increase depends upon the length and breadth 

 of the tubes. In three species the specific increase has been 

 measured. 



Polyporus squamosus (Figs. 1 and 4-7). — In the specimen 

 examined it was found that in the middle of the pileus there were 

 22 tubes to each square centimetre. Each tube on the average 



