CHAPTER V.—COMPARATIVE REVIEW.—HYMENOMYCETES, 307 
basidia have been found close to one another in very different stages of spore- 
formation, or apparently younger basidia among full-grown ones on a small portion 
of the surface of hymenia of moderate age in many Hymenomycetes. It would seem 
as if the basidia which appear to be younger were of later formation than those which 
are maturing, and had been subsequently introduced between them; in other words, 
that an intercalary growth of the hymenium had taken place by insertion of new 
elements, and the fact first mentioned of the juxtaposition of basidia in different 
stages of maturity supports the idea of such intercalation. It is true that the first 
formation of the basidia may have advanced strictly from the middle to the 
margin of the hymenium, and only the last differentiation and maturation have 
proceeded in a different direction; and the apparently young basidia may only have 
been paraphyses which look like basidia. More thorough investigation of this point 
is desirable. 
In many long-lived Hymenomycetes the hymenial surface, which remains simple, 
enlarges in each successive period of vegetation by the growth both of the whole 
pileus and of the separate projections of the hymenium, the growth being in the 
direction of the margin, as was described above on pages 56, 57. In old Polypori 
which come particularly into consideration in this connection (P. fulvus and 
P. igniarius and Trametes Pini) the older parts of the tubes formed in the earlier 
years of the growth of the plant become stopped up, according to Hartig’s detailed 
account, by a dense formation of hyphae in proportion as the tubes themselves 
increase in length at the margin. The hyphae proceed from subsequent ramifications 
of the hyphae of the wall of the adjoining tube, on which the peculiarly short 
and very short-lived basidia performed their office a long time before and then dis- 
appeared. 
But a certain number of long-lived Hymenomycetes renew their hymenium in 
successive periods of vegetation even on the same portion of its surface. A part only 
of the parallel extremities of the hyphae which form the hymenium in the first period 
developes into basidia or into cystidia also. Others, which do not differ in structure 
from young commencing basidia, do not arrive at the formation of spores, but 
remain capable of development and ramify and grow out in the next period of 
vegetation beyond the first hymenial surface to form a new hymenium upon it, which 
is similar to.the first and covers it everywhere. The old basidia and even their ripe 
spores, and in a very striking way the acutely conical hairs or cystidia where there 
are any, are overgrown by the new layer and enclosed in it. The process may be 
repeated from period to period, from year to year. In this way distinct layers 
corresponding to the years, or to shorter spaces of time periodically repeated, are 
formed in hymenia by the remains of the basidia and hairs of each period; a few only, 
according to Hartig, in Trametes Pini; the same observer counted 5-8 in Hydnum 
diversidens, in Thelephora perdix as many as 20; I myself found as many as 6 on 
not particularly old specimens of Corticium quercinum. 
The question of ‘gonidial formations’ reported in some Hymenomycetes will be 
deferred till section XCII in order to avoid repetition. 
