12 Miss Robertson, Notes on the Anatomy of 



thick-walled lignified cells. The phloem is a thin-walled homo- 

 geneous tissue, and is thus markedly distinguished from that of 

 the stem with its noticeable fibres. There is a good deal of 

 centripetal xylem and a little centrifugal, the protoxylem is 

 equally in contact with both. This is generally the case, too, in 

 the pinna, but here it occasionally happens that the whole or a 

 part of the centrifugal xylem is slightly separated from the proto- 

 xylem. It may be worth while to recall in this connection that, 

 according to Scott 1 , in the foliar organs of Cycads it is more usual 

 to find the protoxylem elements in contact with the centripetal 

 part of the wood, but that there are numerous exceptions to this 

 rule. He mentions Macrozamia heteromera, amongst others, as a 

 case in which the protoxylem of the pinna is equally in contact 

 with both parts of the wood. 



According to de Bary 2 mucilage canals do not enter the pinnae 

 of Cycads except in Dioon, Encephalartos, and Stangeria ; in 

 Encephalartos they alternate with the veins. This forms an ana- 

 tomical distinction between Encephalartos and Macrozamia, two 

 genera which Worsdell 3 regards as shewing such close resemblance 

 as to make it probable that they will at some future time be 

 united into one. The distinction, however, is not so absolute as 

 might appear from de Bary's statement, for as a rule in Macrozamia 

 heteromera three bundles enter the pinna accompanied by a 

 mucilage duct, which dies out before the leaflet divides into two, 

 so that it only runs in the latter while it has the appearance of a 

 branch of the rachis, and before it has really quite attained its 

 flattened leaf-like character. Vetters 4 says that in Macrozamia 

 a single bundle is given off into the pinna which divides in the 

 base ; but in our species the division appears to take place before 

 the bundle enters the leaflet at all. Kraus 5 describes the peculiar 

 pit-connections of the mesophyll of Cycadean leaves, and figures 

 separate cells to shew the structure ; de Bary 6 mentions that the 

 pitted areas turn brown with Schultze's solution and deep red 

 with solution of anilin. Macrozamia heteromera exhibits these 

 pit-connections remarkably well. The spongy mesophyll cells are 

 only in contact by comparatively small roundish areas, which are 

 lignified and pitted (Fig. 2). By comparison of transverse and 

 tangential sections of the leaf it is easy to see that these cells lie 

 with their greatest diameter in a direction at right angles to the 

 long axis of the leaf, and so form connections stretching between 



1 "The Anatomical Characters presented by the Peduncle of Cycadaceae," 

 Annals of Botany, Vol. xi. 1897, p. 404. 



2 Comparative Anatomy of Phanerogams and Ferns, English Trans. 1884, p. 441. 



3 Trans. Linn. Soc. Vol. v. Part xiv. 1900, p. 452. 



4 Loc. cit. p. 13. 



5 " Ueber den Bau der Cycadeenfiedern," Prings. Jahrb. iv. p. 326. 



6 Loc. cit. p. 117. 



