12 Proceedings of the Royal Irish Academy. 



history of the peculiar trees, here regarded as hybrid seedlings of the second 

 generation on account of their botanical characters, is obscure. They may 

 ultimately prove to be identical with young seedlings of P. acerifolia, which 

 are now growing at Kew and Glasnevin, when these in after years acquire 

 adult foliage and bear fruit. This would be a positive proof of their liybrid 

 origin. 



In the present paper the results of an investigation into the botanical 

 differences of the two parent planes and their various hybrids will be detailed. 

 Before doing so, it is desirable to give some account of the genus Platanus. of 

 which six living species are known, concentrating our attention on the foliage 

 and fruit, the chai-acters mainly studied by us. 



Chamcifrs of the gouts Platanus. — In all planes, the leaves are alternate, 

 simple, stalketl, pahnately o-o-7 lobed : mai-gin entire or with minute or 

 coarsely sinuate teeth ; venation pseudo-palmate with three or five main 

 nerves ; base of the blade cordate, truncate, or cuneate. Buds concealed in 

 the funnel-shaped base of the leaf-stalk. Stipules two, united into a tube 

 embracing the twig above the insertion of the leaf, thin and scarious on 

 flowering shoots, broad and leafy on vigorous barren branchlets. Flowei-s 

 monoecious, in uni-sexual heads. Fruiting heads globose, each bull made up 

 of numerous closely jacket! achenes ; style persistent or breaking ofl' from 

 the top of the achene. 



Tlie differences in the species are not great. Each occupies a distinct 

 r^on of the earth's surface ; and the modifications exhibited by the leaves 

 and fruits are prol>ably adaptations, fitting each species to the climate and 

 soil of the territory in wliich it grows. 



The leaves show specific diflerences as regards the woolly mat of hairs on 

 their surface, and in respect of the depth of their lobing. In all the species, 

 the leaves, on opening, are densely coveretl with wool ; but as the season 

 advances, the wool either disappears completely or pei-sists to a lesser or 

 greater extent. In P. orienialis, it practically vanishes, while in P. occidtntalis, 

 it persists along the main ner\'es, and on the stalk. The hybrids resemble 

 one or other species as regard.s this character. The Oriental Plane, the leaves 

 of which become bare and unprotected, gi-ows in the wild state as a rule in 

 wet places beside streams or springs, and is amply supplied with water. In 

 the four species which are natives of the arid climate of Arizona, California, 

 and Mexico, the woolly covering remains on the surface of the leaf. In other 

 words, the greater the demand of the tree for water, the more complete 

 is the protection afforded against transpiration by the pubescence of the 

 leaf. 



