260 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



mycelium : the outermost of these spores i.e. 

 those which form a compact layer close beneath 

 the epidermis remain barren, and serve as a kind 

 of membrane covering the rest (Fig. 39, /). It is 

 this membrane which protrudes like a blister from 

 the tissues. The hyphae of the fungus are seen 

 running in all directions between the cells of the 

 leaf-tissue, and as they rise up and form the 

 vertical chains of spores, the pressure gradually 

 forces up the epidermis of the leaf, bursts it, and 

 the mass of orange-yellow powdery spores protrude 

 to the exterior, enveloped in the aforesaid membrane 

 of contiguous barren spores. If we examine older 

 cecidia (Fig. 38, b) it will be found that this mem- 

 brane at length bursts also, and the spores escape. 



Similar sections across a spermogonium exhibit a 

 structure which differs slightly from the above. Here 

 also the hyphae in the leaf turn upwards, and send 

 delicate branches in a converging crowd beneath the 

 epidermis ; the latter gives way beneath the pressure, 

 and the free tips of the hyphae constrict off extremely 

 minute spore-like bodies. These minute bodies are 

 termed Spermatia, and I shall say no more about them 

 after remarking that they are quite barren, and that 

 similar sterile bodies are known to occur in very many 

 of the fungi belonging to this and other groups. 



