50 FUNGUS-FLORA. 



development in the sub-group is not the outcome of a new 

 initial force, but simply the continuation of epinasty, which 

 kept type (a) all pressed 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 

 >f the substratum, it is only necessary to place a prostrate 



branch with the plant growing on it, as in the (a) type, in a 



" " How (b] 

 type; and microscopic examination will clearly reveal the 



vertical position, when further development will follow (6) 



epinastic curvatures of the hyphae in the thallus, as in the 

 section of Stereum hirsutum given by De Bary.* The above is 

 a remarkable illustration of a new type of structure due 

 entirely to surroundings, (c) In type (6) three-fourths or 

 more of the plant is usually attached to the substratum, and 

 this is more especially the case when growing on the side of 

 a prostrate trunk or large branch, where the side presents, 

 compared to the size of the plant, a practically flat vertical 

 surface, which appears in some way to neutralise, to a great 

 extent, the epinasty of the plant ; whereas, when growing on 

 the side of a small prostrate branch, when the antagonistic 

 flat surface is reduced to a minimum, the plant often becomes 

 free soon after the commencement of growth, the upper, free 

 horizontal portion still continuing to develop in a more or 

 less circular manner, which results in a structure that can be 

 understood by comparing it to a reniform leaf attached by a 

 short, flat petiole to the branch, the lamina being free, more 

 or less depressed in the centre, and incurved at the margin. 

 This stage illustrates the origin of a central stem and um- 

 brella-shaped pileus, which is perfected in (d) by the two 

 lateral lobes becoming united behind, which results from the 

 plant growing from a point where it is free to expand equally 

 on every side from a short stem-like base. 



Passing to the highest order of the Hymenomycetes, the 

 Agaricineae, we meet with the same sequence of sporophore 

 development. In the genus Pleurotus, such simple, stemless 

 forms as P. applicatus, Batsch, illustrate the (a) type, being 



* ' Fungi, Bacteria and Mycetozoa.' Engl. Ed., p. 53, fig. 23. 



