ON THE STRUCTURE OF THE TISSUES OF CYCADE^. 169 



which is generally on the perpendicular sides of the vessel. In other 

 cases the vessels are distinctly reticulated, and they then exhibit a 

 striking analogy to the dotted cellules in Cycas revoluta. All these 

 modifications are frequently to be observed in the same vessel in Zamia 

 furfuracea and pumila, a fact which affords conclusive evidence of the 

 accuracy of the theory advanced by Meyen, which refers the spiral, 

 annular, reticulated, and dotted vessels to a common type. The dots 

 and stripes are evidently the thinnest portions of the tube, being most 

 probably parts of the primitive membrane remaining uncovered by the 

 matter subsequently deposited on the walls. 



The cellular tissue of Cycadece consists of tolerably regular paren- 

 chyma, composed of prismatic, six-sided cellules. In the species of 

 Zamia and Encephalartos, so often referred to, the walls of the cellules 

 appear to be of a uniform thickness and transparency, and destitute 

 both of dots or marking ; but in the adult fronds of Cycas revoluta, a 

 different structure presents itself, for the walls of the cellules are fur- 

 nished with numerous elliptical, obliquely transverse dots or spaces, 

 where the membrane is so exceedingly delicate and transparent as to 

 give the cellules the appearance of being perforated by holes, the in- 

 tervening spaces being covered by incrustating matter, disposed in the 

 form of confluent bands, which, when viewed under the microscope, 

 resemble a kind of net- work. The dots or spaces uncovered by incrusting 

 matter, are generally of a large size, and occur more particularly on the 

 vertical sides of the cellules, a band usually running along the middle 

 of the two opposite sides. The bands vary in breath, as do the dots, 

 and they not unfrequently exhibit minute transparent points or spaces, 

 where the solid matter forming the band shows a tendency to separate. 

 The extreme delicacy and transparency of the dots or spaces of what- 

 ever size, appear fully to prove that they are parts of the primitive 

 membrane of the cellule, which are uncovered by the incrusting matter. 

 A solution of iodine will be found of great service in determining the 

 actual existence of the membrane at those parts ; for although it does 

 not materially alter its colour, it tends very much to dimmish its trans- 

 parency and renders it distinctly visible, so as to leave no doubt that 

 the spaces are not openings. The bands are evidently the result of a 

 partial lignification ; and indeed no better example can be offered than 

 Cycas revoluta to illustrate and confirm the correctness of the views ad- 

 vanced by Schleiden as to the origin of the bands and fibres in the cel- 

 lules and vessels of plants.- Being anxious to ascertain whether the 

 bands exist at an early period, the author had recourse to the examina- 

 tion of a young undeveloped frond, about two weeks old, and he was 



