32 



PRINCIPLES OF PLANT-TERATOLOGY. 



where an obvious transition is found ; in fact in many 

 cases there obtains a structure which is clearly com- 

 posed neither of gills nor of pores, but is transitional 

 between the two. Boudier, in speaking of his abnormal 

 Cortinarius (above described) states that the change 

 to the alveolar condition is due to the protection which 

 is required for the spores when situated on the upper 

 surface, which a gill-structure would never provide. 

 This is, doubtless, the reason why this structure 

 obtained, and still obtains, in the primitive types which 

 exhibit it, but it can hardly be the explanation of its 

 presence in the abnormal Agarics, whigh is — a rever- 

 sion to that primitive condition. The Polyporacese 

 and Hydnacese represent a more ancestral type, as 

 regards this fertile tissue, than the Agarics. The 

 latter have completely lost the original alveolar or 

 pore-structure. A phenomenon which demonstrates 

 that that is the primitive and the gill-structures 

 are the more recent is shown in the fructifications 

 of Trametes lobata and certain species of Dsedalea. In 

 the development of the cap, the pore-structure, in 

 typical forms, is first produced, later on a transitional 

 structure, and finally (forming the marginal portion of 

 the whole cap) gill-formation entirely supervenes. 

 Here we have, in the ontogeny of a member of the 

 Polyporacere, the evolutionary stages which have been 

 passed through in the phylogeny with the Agarics as 

 the result. Some individuals of one and the same 

 species of Dxdalea have pore-structure only, others 

 have gill-structure only, throughout the lower surface 

 of the cap ; others, again, show transitions between 

 the two. 



In the evolution of the Agaricinea3 we may therefore 

 postulate three changes as having taken place : 



1. The production of the horizontally-extended 

 flattened cap-form of fruit from the original cylindric 

 dome-shaped form. 



2. The relegation of the hy menial tissue to the lower 

 surface of the cap. 



