388 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



converted into a complete ring of very hard, white, woody tissue. 

 The three groups of vessels now form a complete cylinder, but thinner 

 above than below, closely similar to that of the stem." 



The upper surface of petioles is frequently more or less concave. 

 The edges thus form flanges producing a structure similar to the stays 

 of Fox's umbrellas. It is characteristic of large leaves, as of the 

 Umbeiliferae. By extending the flanges round the stem, a complete 

 sheathing base is secured. This is particularly characteristic of 

 palms, so that the petiole may have more " purchase " in supporting 

 the sometimes enormous weight of the blade. 



With regard to the arrangement of the supporting " ribs " and 

 " veins " of a leaf-blade, no one can fail to see how they are distributed 

 to the best advantage, so as to support the extended surface for the 

 sunhght. But leaves thus displayed are subjected to strains by the 

 wind, which they must resist, and to avoid laceration transversely 

 the midrib and lateral veins are so distributed as to secure the greatest 

 strength. It will be noticed that the veins stand at acute angles, from 

 the midrib, so that if a hne be drawn transversely it falls across, 

 it may be, from four to six veins besides the midrib. 



If the veins are at right angles less strength is secured. As a 

 consequence, the leaves of bananas are always tattered by transverse 

 tearhigs from the edges to the midrib. 



One method of obviating the chance of tearing is to break up a 

 simple leaf into a compound one ; when that is the case, the midribs 

 of the leaflets stand at right angles to the main rib, as in a rose. 

 Another advantage seems to arise. In a simple leaf the petiole is 

 just long enough for its requirement, but when it becomes a compound 

 leaf, as is easily seen in a blackberry or raspberry, then the petiole 

 elongates and is able to carry from three to five or seven leaflets. 

 The further advantage of less obscuration of light for the under- 

 lying leaves is secured, as \yell as the total area of the blades being 

 far greater than that of one only. 



