48 Linncean Society. 



fusca laxiuscula, vix nigosa, in alam fusco-membranaceum semen 

 cingentem expaiisa. Einbryo parvus, teres, rectus, pi-ope basin albu- 

 minis copiosi, radicula juxta hilum, cotyledonibus parvis. 

 Herba perennis, uliginosa. Folia radicalia ; petiolus tubuloso-amphorce- 

 fortnis, ore obliquo margine subrevoluto. Scapus erectus, apice simpli- 

 citer racemosus, glaber. Floras nutantes, albi v. pallide roxei. 

 1. H. nutans. 

 Read a paper, entitled " On the Structure of the Tissues of Cy- 

 cadeee," By D. Don, Esq., Libr. L. S., Prof. Bot. King's College. 



In ConifercB the structure of the stem presents the ordinary appear- 

 ance of dicotyledonous trees ; the annual layers are distinctly 

 marked, and there is a regular bipartition of each into wood and bark 

 (liber) ; but in Cycadece no bipartition takes place of the fibro-vascular 

 bundles, which in that respect resemble those of monocotyledonous 

 plants, and the differences otherwise are very striking, Cycas having, be- 

 sides a large central 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 Conifera, and which distin- 

 guishes them as well from Cycadea as from every other family, is 

 the remarkable uniformity of their woody tissue, which consists of 

 slender tubes, furnished on the sides parallel to the medullary rays 

 with one or more rows of circular or angular dots ; but in Cycadee 

 no such uniformity is observable, their tissue, as in other phsenoga- 

 mous plants, consisting of two kinds of vessels, namely of slender 

 transparenttubes, without dots or markings, and of dotted, reticulated 

 and spiral vessels, which are capable of being unrolled. The former are 

 identical with the fibrous or woody tissue, whilst the latter, which form 

 a part of each bundle, can only be compared to the strictly vascular 

 tissue of other plants. These dotted vessels in Cycadea bear a con- 

 siderable resemblance to the vessels of Coniferce, and especially to 

 those of Dammara and Araucaria, from the dots being disposed in 

 rows, and confined to the two vertical sides of the vessel only, and 

 they are moreover alternate, as in the two genera just mentioned. 

 In Cycadece, however, the dots present much less regularity in 

 number and size than in Conifera, not only in different vessels of the 

 same bundle, but in diflferent parts of the same vessel, forming one, 

 two, three, four, and five rows ; and they are not always confined 

 to the vertical sides, but appear in some cases to follow the entire 

 circle of the vessel. Their form is oblong, or elliptical, in Cycas re- 

 valuta, circinalis, glauca, and speciosa, Zamia furfuracea and pumila, 

 as well as in Encephalartos horridus and spiralis; but they are 

 sometimes longer, narrower and nearly linear, giving the vessel 



