ABSTRACTS OF FOUR LECTURES TO STUDENTS. 



515 



for heat by fermentation for the eggs. Stipules not infrequently secrete 

 honey as well as other parts of plants, rendering them attractive to ants 

 and other insects. Such occur on Vetch, species of Begonia, &c* 



II. The Composition of Leaves. 



Leaf Composition. — We must now consider how leaves are made. 

 A typical leaf consists, visibly, of three parts — the petiole or stalk, the 

 lamina or blade, and a pair of stipules. Any one or two of these may 

 be wanting. Hence this is the most convenient method of regarding a 

 leaf.t 



The Petiole. — As this, usually a rod-like structure, has to carry the 

 weight of the blade, which, in the case of Palms, may be enormous, 

 it must be proportionally strong, especially at the base for leverage. It 

 has, therefore, a strong cord or fibro-vascular bundle, running along the 

 underside together with two or more smaller ones above it, a section 

 through them showing that they are in a horseshoe-shaped curve. 



When the cords enter the blade they send off branches forming the 

 framework or " skeleton " of the blade, so as to display as large a surface 

 as possible to the light. They thus constitute the supporting framework. 



We have seen that the cords which enter a petiole issue directly out 

 of the stem- cylinder, the largest and lowermost from the point immediately 

 behind and below the point of insertion of the petiole. The others issue 

 from more distant points and travel round till they enter the petiole at the 

 sides of the first. 



If there be stipules, their cords are given off from the lateral ones of 

 the petiole. This is also the case in the so-called "adnate" stipules of 

 the Rose. 



When the base of the petiole spreads and makes a "sheath" of less 

 or greater extent, sometimes completely investing the stem, as in Palms, 

 it thus secures a greater " purchase " to sustain the weight of the blade. 

 In some Palms the innumerable fibres cross each other, adding great 

 strength to the petiole. 



Such only fulfils a universal function in plants as in animals : that 

 when strains are " felt " nature at once builds up tissues to meet them. 



In large leaves, as of Umbellifers, it will be noticed that the two upper 

 cords of the petiole form a pair of flanges. These supply much additional 

 strength to resist the strain caused by the weight of the blade, and so 

 prevent fracture. 



Fox's steel umbrella- stays are made on the same principle, but the 

 flanges are on the underside in order to resist the strain when the wind 

 would blow the covering inside out, as well as due to curvature when the 

 umbrella is open. 



It is, therefore, of common occurrence to find petioles channelled along 

 the upper side. 



In some, however, the cords form a complete cylinder precisely as in 

 stems. Such occurs in Maples and Horse-chestnut and in that of the 



* a great many interesting details on stipules will be found in Lord Avebury's 

 (Sir J. Lubbock) Buds and Stipules (International Scientific Series, vol. lxxxvi.), 



f Lord Avebury adds the "leaf-base," as of a Rose-leaf: but I do not think this 

 is necessary from anatomical considerations. 



