EXCEPTIONAL MODES OF DEVELOPMENT. 143 



enter the leaves, and it is only inside the leaf -stalk that these ramifications 

 are met by branches from the peripheral network. The bundles of the 

 roots arise only from the peripheral bundles, but those of leaves, as already 

 said, receive branches from both axillary and peripheral bundles. Two 

 thick brown plates {sclerotic prosenchyma) lie between the inner and 

 outer systems of bundles, and are only separated from one another at the 

 sides by a narrow band of parenchyma. They are often joined on one side 

 or even on both, in the latter case forming a tube which separates the 

 two systems of bundles. (Hofmeister.) 



Apogamy. Apospory. In rare cases, e.g., in Pteris cretica^ the ordi- 

 nary alternation of generations in the life-cycle of ferns is abbreviated by 

 the omission of the sexual process, and the immediate vegetative outgrowth 

 of the sporophore from the prothallium (apogamy). In other cases there 

 is an omission of the spore stage, and immediate vegetative development 

 of the oophore from the frond {apospory). (cf. Farlow, Quart. Journ. 

 Mic. Science, 1874; De Bary, Botan. Zeitimg, 1878; Druery, etc., Journ, 

 Royal Mic. Soc, 1885, pp. 99 and 491.) 



