SPORE-DISCHARGE IN THE HYDNEAE 151 



pileus brings the long axes of all the partially developed spines 

 into positions which are more or less perpendicular to the surface 

 of the earth. After the expansion of the pileus, the spines continue 

 their growth in length and become positively geotropic. By 

 responding to the stimulus given by the earth's mass, they bring 

 their axes into exactly vertical positions with respect to the earth's 

 surface. In Hydna with centric stipes, the spines, just like the 

 tubes of the Boleti, are brought into their right positions in space 

 by two adjustments a coarse one consisting of the expansion of 

 the pileus, and a fine one consisting of the reaction of each spine 

 to the stimulus of gravity. The fine adjustment causes the 

 hymenium on the sid?s of the spines to look slightly downwards 

 toward the earth in the same manner as the hymenium on the 

 sides of gills ; and a median vertical section through a straight 

 spine of Hydnum imbricalnm, H. septentrionale (Figs. 53, 54), 

 H. erinaceus (Fig. 55), H. coralloides, etc., resembles in its wedge- 

 shaped form a transverse vertical section through a gill of Psalliotc, 

 campestris or an Amanita. Spines, gills, and hymenial tubes on 

 the under side of a pileus are all highly efficient means of increasing 

 hymenial surface without loss of compactness for the fruit-body 

 as a whole ; and in general they are so constructed that their free 

 surfaces, and therefore also their hymenial coverings, look either 

 slightly downwards toward the earth or horizontally. Thus 

 provision is made for interspinal, interlamellar, and intratubular 

 spaces of such a form as to favour the escape of the spores when 

 these have been shot outwards from the hymenium and are falling 

 vertically. 



The basidia of the Hydneae discharge their spores in the same 

 manner as other Hymenomycetes. Just before the discharge of 

 a spore, a drop of water is excreted at the hilum and, when dis- 

 charge takes place, the spore is violently shot forward into an 

 interspinal space to a distance of about 1 mm. After travelling 

 this distance, the spore makes a sharp turn from a horizontal to 

 a vertical direction and then falls slowly downwards with a steady 

 terminal velocity between the spines. The sporabolas of a Hydnum 

 and of a Psalliota are identical in form. 



The excretion of a drop of fluid at the hilum of a spore a few 



