CQVRSE OF THE BUNDLES IN THE STEM. 263 



section through the median planes of the leaves, Fig. 1 1 8. But the assumption of a 

 tangentially-perpendicular course only holds good for bundles which are also radially 

 perpendicular. As Meneghini^ first asserted, and Mohl also conceded (Verm. 

 Sehriften, p. 160), and as Naegeli proved more accurately, each radially curved bundle 

 runs also tangentially oblique, with a spiral curvature, which is stronger the stronger 

 the radial curvature. Nageli found e.g. that the niedian bundle of a leaf of Chamse- 

 dorea elatior Mart, made i \ revolutions in passing through six internodes : in the 

 sixth it had not quite accomplished half the distance from the centre of the stem 

 to the inner surface of the cortex, on its way outwards. In stems with very short 

 internodes and closely packed bundles the spiral curvature is at once visible in the 

 transverse section, and very plainly in the bundles of the stem of Xanthorrhoea, 





, FIG. 117. — Mohl's scheme of the vascular system of 

 Monocotyledons. Successive leaves, or rather sue. 

 cessive nodes, are numbered in order. 



Fig. 118. — Scheme of the vascular system in the Palm- 

 type, assuming that the leaves alternate in two rows, and 

 embrace the stem. Successive leaves numbered in order. 

 m median bundle, 



which pass almost horizontally to the middle of the stem : the peculiar appearance 

 so often noted ^ in transverse sections of this plant depends upon the above 

 peculiarities. 



Finally, many variations, of direction from that hitherto described as uniform 

 may occur in the course of a bundle, such as curvatures outwards and inwards, &c., 

 which never appear to be constant. 



The above description holds both for the preponderating number of cases, in 

 which those bundles which penetrate deepest reach the middle of the stem, and also 

 for those where, as in the stems of grasses which become hollow, a broad central 

 part (pith) remains free from bundles. Where the internodes are short, as e.g. in 

 the well-known preparations by maceration of the stems of Dracsena Draco, the 

 course can easily be recognised in the main features. Where the internodes are 



' Ricerche snlla struttura del caule nelle piante monocotiledoni, Padova, 1836. 



^ De Candolle, Organ ographie, I. Tab. VII, VIII.— Schleiden, Grundziige, 3. Aufl. II. p. 160. 



