50 PROFESSOR THISELTON DYER, 



On Other places, on the contrary, the ciliated endothelium 

 is also to be met with on the surface between the stomata 

 extensively distributed in the form of trabeculoe and knots 

 of proliferating ciliated endothelial cells raised above the 

 surface. 



On the Structure of the Stem of the Screw Pine. By 

 W. T. Thiselton Dyer, B.A., B.Sc, Professor of Botany, 

 Royal College of Science for Ireland. (With Plate V.) 



Species of the genus Pandanus, or screw pines proper, 

 are a conspicuous element in the vegetation of the East 

 Indian islands. Representatives of them are to be found in 

 all large collections of tropical plants; yet, notwithstanding 

 this, the few notices of their structure to be met with in 

 books are meagre and incomplete in the extreme. 



Looking at the anatomy of their vegetative organs, screw 

 pines belong to the same type of arborescent monocotyledons 

 as Yncca and Draccena. These differ from single-stemmed 

 palms in producing a crown of branches which increases in 

 size and complexity as long as the plant lives. Under these 

 circumstances' it is evidently a mechanical necessity that 

 there should be some provision for the enlargement of the 

 primary stem of the plant, so as to afford an adequate support 

 for the increasing weight of structures which it has to uphold. 

 In palms the single stem has to bear little more than its own 

 weight, and as it tapers upwards each successive increment 

 of growth to the extremity probably adds less to the burden 

 than the increment which preceded it. But it is qj.iite other- 

 wise when the primary stem breaks up into branches ; each 

 of these continues to divide, the number of growing extremi- 

 ties is multiplied, and every successive addition to the weight 

 of the whole crown adds a heavier load than the last. 



The fact that, at any rate, Dracana possesses a provision 

 for continuous additions to the circumference of the stem has 

 long been known. This, independently of others, is a suffi- 

 cient reason for abandoning the use of the term E}idoge7is to 

 describe monocotyledons ; such a stem as DraccBna possesses 

 is as much exogenous as that of an oak. Millardet^ has care- 

 fully studied the circumferential growth both of Z^rac^ewa and 

 Yucca ; he finds that new fibrous bundles make their appear- 

 ance in a cambium layer immediately exterior to the mass of 

 original bundles, which undergoes itself no alteration. In 



I 'Mem. de la Soc. Imp. d. Sc. Nat. d. Cherbourg,' 2e s^r., i, pp. 319—352. 



