762 PHYSIOLOGY. 



want an absolutely certain basis, which cannot be obtained 

 without observing the internal life of the parenchyma of the leaf 

 and of its products. This ought to remain the object of a true 

 history of the development of leaves ; for at present their suc- 

 cessive transformations only have been observed." 



The following is an abstract of Trecul's conclusions, as given 

 by Balfour in his " Class Book of Botany :" — 



" All leaves originate in a primary cellular mammilla, with or 

 without a basal swelling, according as they are to have sheaths 

 or not; they are developed after 4 principal types: 1, the 

 centrifugal formation, from below upwards ; 2, the centripetal 

 formation, from above downwards; 3, the mircfZ formation ; and, 

 4, the farallcl formation. The centrifugal development may be 

 illustrated by the leaf of the Lime-tree, which begins as a simple 

 tumour at the apex of the stem. This tumour lengthens and 

 enlarges, leaving at its base a contraction which represents the 

 petiole. The blade, at first entire, is soon divided from side to 

 side by a sinus. The lower lobe is the first secondary vein. 

 The upper lobe is divided in the same manner 5 or 6 times, 

 forming as many secondary veins. Sinuosities then appear in 

 the lower lobe, indicating the ramifications of the lower vein ; 

 and, finally, fresh toothings appear corresponding with more 

 minute ramifications. Thus the various veins in the leaf of the 

 Lime-tree are developed like the shoots of the tree that bears 

 them, and the toothing does not arise from cells specially 

 adapted for that purpose on the edge of the leaf, as Mercklin has 

 supposed. The hairs on the under siu'face of the leaf are also 

 formed from below upwards. 



" Leaves developed centripetally are equally numerous with the 

 preceding ; of this sort are the leaves of Sanguisorba officinalis, 

 Rosa arvensis, Ccphalaria procera, &c. In them the terminal 

 leaflet is first produced, and the others appear in successive 

 pairs downwards from apex to base. The stipules are produced 

 before the lower leaflets. All digitate leaves, and those with 

 radiating venation belong to the centripetal mode of formation 

 as regards their digitate venation. 



" In some plants, as Acer, the two preceding modes of develop- 

 ment are combined. This is called mixed formation. In Acer 

 platanoidcs the lobes and the midribs of the radiating lobes form 

 from above downwards, the lower lobes being produced last, but 

 the secondary venations and toothings are developed like those 

 of the Lime-tree. In Monocotyledons we meet with the parallel 

 leaf formation of Trecul. All the veins are formed in a parallel 

 manner, the sheath appearing first. The leaf lengthens espe- 

 cially by the base of the blade, or that of the petiole when 

 present. 



" Leaves furnished with sheaths, or having their lower portions 

 protected by other organs, grow most by their base ; while those 



