18 THE TRANSVERSE SECTIONS OF PETIOLES OF EUCALY'PTS 



The above table shows a contrast in various respects between the young square 

 stem and the midrib of the young sessile leaf. 



In the stem — 1st. The epidermis is thinner. 



2nd. The number and size of cortical cavities is greater. 



3rd. The hard bast is much denser. 



4th. The wood forms a square, and not a crescent. 



The width of the vessels is about the same in both. The upright growing stem 

 and the lateral growing leaf are necessarily unequally acted on by various forces, 

 hence the symmetrical and stronger F. V. B. of the stem, as compared with that of 

 the leaf. 



If the comparison is made between ihQ young and the old leaf, with its twisted 

 petiole, there are a few striking differences along with general agreement ; but it 

 will be seen that the differences are simply those between a young and tender organ, 

 and the same arrived at maturity. In the old leaf, the epidermis is about twice as 

 thick, the cortical cavities are increased and immensely enlarged, the hard bast is 

 now strongly developed, and the vessels are a trifle larger. 



The twisting of the petiole seems to have produced no noticeable difference in the 

 general arrangement of the tissues, particularly the vascular. The twisting 

 of a leaf-stalk round a support, in the case of leaf-climbers, has been shown by 

 Darwin to increase the growth. The pressure here has acted as a stimulus, but it is 

 worthy of note that while in the free petiole the bundles form an open curve, in the 

 clasping petiole they form a closed cylinder. 



The mere twisting of the petiole in Eucalypts has not produced any curving of 

 the bundles greater than that met with in the young and sessile leaf, and even in 

 those cases where the horns of the crescent most closely approach, as in fictfolia and 

 calophylla, the leaf-stalk is less twisted than in ordinary cases. 



Further, the statement made by Mr. Darwin, on the authority of Dr. Masters, 

 that " the semi-lunar band of vessels is confined to petioles channelled along their 

 upper surface," is not countenanced by these researches, for it is in the most 

 channelled leaf-stalks that the w^ood curve most approaches the complete ring. 



* Darwin, " On the Movements and Habits of Climbing Plants," p. 43. 



