110 MR. GEORGE MASSEE ON THE THELEPHORES, 
lution as occurring in the Hymenomycetes, illustrated by Stereum 
hirsutum, Fr., one of the Thelephorex :—(a) The most primitive 
type, as explained above, is where the sporophore is spread out 
as a thin layer attached to the substratum by the whole of the 
under surface, the upper surface being covered with the hymenium 
(Pl. XLV. fig. 1). In many of the simpler Thelephoree this 
mode of growth is permanent, independent of the direction of the 
substratum ; but in species like Sterewm hirsutum, which may be 
described as inclined to *sport, or, more correctly, where epi- 
nasty, the cause of the (a)-type of sporophore, is strongly mani- 
fested, the above mode of growth occurs when developing on a 
broad horizontal substratum. (5) When the substratum is 
vertical, which may be the side of a prostrate trunk, or an erect 
one, growth commences as in type (a), and after extending from 
a centre for some time, and assuming a more or less circular out- 
line, the uppermost margin becomes free and continues to grow 
away from the substratum and at right angles to the attached 
portion. In this type we get the first transition from the superior 
to the inferior hymenium imperfectly indicated (Pl. XLV. fig. 2); 
and it is interesting to remember that the first step towards the 
inversion of the hymenium, itself the most pronounced result of 
development in the subgroup, is not the outcome of a new initial 
force, but simply the continuation of epinasty, which kept type 
(a) adpressed to the horizontal substratum. When growing in 
what may be termed an unnatural position, the dominant directive 
force, epinasty, directs the plant along the old hereditary lines, 
and as soon as possible the horizontal position is resumed with the 
free margin incurved. To prove that this change of direction of 
growth is due to the position of the substratum, it is only necessary 
to place a prostrate branch with the plant growing on it, as in the 
(a)-type, in a vertical position, when further development will 
follow (2)-ty pe; and microscopic examination will clearly reveal 
the epinastie curvatures of the ‘hyphe in the thallus, as in the 
section of Stereum hirsutum given by De Bary*. The above is a 
ran Mustration of the evolution of a new type of structure 
more of the rroundings. (c) In type (b) three fourths or 
more of the p ant is usually attached to the substratum, and this 
a De tn he case when growing on the side of a prostrate 
size of the plant, a » M cre the side presents, compared to the 
Ma ^» a practically flat vertical surface, which appears 
ngi, Bacteria, and Mycetozoa,’ Engl. ed. p. 53, fig. 23. 
