ON THE STRUCTURE OF THE TISSUES OF CYCADE.E. 167 



part of the dispersion be derived from the inflexion by the irregularly 

 situated particles at the surface ? 



5. As the combination of bismuth with some other metals adds much 

 to their fluidity in the melted state, would not the oxide of bismuth 

 probably add much to the fluidity of glass in the melted state ? 



6. If we can render glass very fluid in the melted state, and cast 

 lenses in finely polished moulds, is it not highly probable that the sepa- 

 rate particles will arrange themselves by mutual attraction, much more 

 regularly than the grinding and polishing can leave them ? And may 

 we not in this way hope to lessen the dispersion, or at least its irregu- 

 larity ? 



Further, it is said that the alkalies render glass liable to a slow de- 

 composition. If we could make transparent glass of alumina and bis- 

 muth, I have reason to believe that we should obtain great refractive 

 power, very little dispersion, and great fluidity in the melted state, 

 which are important desiderata ; but it is highly probable that any 

 combination with alumina would produce an opaque enamel. He con- 

 cludes by saying that he has not heard of any experiments made for 

 these specific purposes. 



XXVIII. ON THE STRUCTURE OF THE TISSUES OF CYCADE2B.* 



By the late David Don, Esq., Professor of Botany, King's College, 



London. 



IN Conifers the structure of the stem presents the ordinary appearance 

 of dicotyledonous trees ; the annual layers are distinctly marked, and 

 there is a regular bipartition of each into wood and bark (liber) ; but in 

 Cycadete no bipartition takes place of fibro- vascular bundles, which in 

 in that respect resemble those of monocotyledonous plants, and the dif- 

 ferences otherwise are very striking, Cycas having, besides a large cen- 

 tral pith, several thick concentric alternating layers of cellular and fibro- 

 vascular tissue ; and in Zamia and Encephalartos, besides the pith, there 

 are only two very thick layers, one of fibro- vascular, and the other, 

 which is also the exterior one, of cellular tissue. The great peculiarity 

 of the Coniferte, and which distinguishes them as well from Cycadete as 

 from every other family, is the remarkable uniformity of their woody 



From the Proceedings of the Linnaean Society, 1840. 



