168 ON THE STRUCTURE OF THB TISSUES OF CYCADE^E. 



tissue, which consists of slender tubes, furnished on the sides parallel 

 to the medulary rays with one or more rows of circular or angular 

 dots ; but in Cycadece no such uniformity is observable, their tissue, as 

 in other phsenogamous plants, consisting of two kinds of vessels, namely 

 of slender transparent tubes, \vithout 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 Cycadece 

 bear a considerable resemblance to the vessels of Coniferce, and espe- 

 cially to those of Dammara and Araucaria, from the dots being dis- 

 posed 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 Cycadea, however, the dots present much less regularity in number 

 and size than in Conifers, not only in different vessels of the same 

 bundle, but in different 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 vessels. Their form is oblong or elliptical, in Cycas revoluta, circi- 

 nalis, 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 the appearance of being 

 marked with transverse stripes. The vessels in all present so much 

 similarity, that no generic distinction can be drawn from them. The 

 dots are always arranged diagonally. The dotted vessels of Zamia fur- 

 furacea and pumila were observed to unroll spirally in the form of a 

 band, presenting a striking resemblance to those of Ferns. The band 

 was found to vary in breadth in different vessels, and was furnished 

 with transverse rows, composed of two, three, or more dots. The coils 

 followed the direction of the dots, and the unrolling was from right to 

 left. In Cycas revoluta dotted vessels frequently occur with a single 

 row of dots ; but, from the circumstance of the dots on both sides being 

 in view at the same time, they are liable to be mistaken as having a 

 double row on each side. Besides the dotted vessels, there occurs 

 throughout Cycadeec another variety, differing but little from the ordi- 

 nary spiral vessel, except in the tendency of the coils to unite. In some 

 vessels the coils are free, and the fibre exhibits frequently, at intervals, 

 bifurcations or narrow loops ; in others, the coils unite at one or both 

 sides, in which case the vessel presents a series either of rings or bars ; 

 and the fibre then is with difficulty unrolled, and it often breaks off 

 into rings, or the bars separate at the point where the coils unite, 



