STRUCTURE OF THE STEM OF THE SCREW PINE. 53 



Pandaniis^ contrary to the usual arrangement in monocoty- 

 ledons^ their walls are decidedly thicker than those of the 

 liber cells. By transmitted light they have a yellowish 

 colour, and collectively they give, viewed with a low power, 

 a darker aspect to the inner portion of each bundle. The 

 wood-cells in the centre of the bundle are often twice as large 

 as the rest in diameter; possibly they may correspond to the 

 " vasa propria " of Mohl. After repeated search I have 

 failed to see anything in the interior of the bundles which 

 could be properly described either as vasa propria or cambi- 

 form cells. 



The vessels which are "surrounded by the wood-cells 

 consist of two large scalariform ducts, each of which is some- 

 times replaced by two smaller ones, presenting an appear- 

 ance as if it had been divided by a transverse septum. Like 

 all scalariform ducts, these are angular in section ; but the 

 angles are not parallel to the axis of the duct, and appear to 

 me, in many cases, to wind spirally round it. Ducts Avith 

 scalariform markings are often stated to be characteristic of 

 the vascular cryptogams. This is not strictly the case. 

 Sachs ^ figures them in Ricinus ; Professor Dickson- has 

 observed them in Smilax; they occur also in the tissues of, 

 at any rate, fossil Cycadea? A few spiral vessels, mucli 

 smaller in diameter than the scalariform, occur on their 

 inner side. 



The inner side of the bundles is, as generally the case, 

 wedge-shaped. In the screw pine it, however, appears 

 frequently to happen that the bundFes are reversed, so that 

 the liber is turned towards the centi-e of the stem. When 

 this is the case it seems generally also to happen that there 

 is more or less complete fusion of two adjacent bundles. 

 More rarely even three are combined in this way — and the 

 resultant effect is very curious — the united bundles having 

 a triangular section with the ducts at each of the angles. I 

 have met with no other stem in which there is any arrange- 

 ment like this. 



The most remarkable feature, however, in the bundles, is 

 the occurrence, at regular intervals round their periphery, 

 of strings of small parenchymatous cells, each of which con- 

 tains an oblong prismatic crystal. In a transverse section the 

 internal reflection of light within the crystals makes the 

 cells containing them appear almost black under a low 

 power. The curious aspect given to the margins of the 



' 'Lehrbuch d. Bot.,' 2ad ed., p. 94. 



" ' Proc. Dub. Mlc. Club.,' vol. i, 193, 



^ Carruthers, 'Trans. Limi. Soc ,' xxvi, p. 097. 



