40 BUDS ON LEAVES. 



process is repeated over and over again, and results in the trees, upon the bark 

 of whicli the Aspleniimi has established itself, being regularly encircled and woven 

 over by fronds, as is shown in fig. 199. The separate fronds of the fern in such 

 circumstances have a strong resemblance to the runners of certain species of 

 Veronica, Ajtoga, and Periwinkle, which have their leaves arranged in two rows. 



Unlike the above cases — viz. the buds of Helwingia borne on special stalks 

 adherent to the leaves, those growing on the cladodes of the Butcher's-broom, 

 and those on the fronds of Ferns, all of which must, in spite of their extreme 

 similarity to epiphyllous buds, be looked upon as cauline — true epiphyllous biids 

 always arise from cells of a true leaf and have no connection with adjacent axes 

 beyond that involved in the fact of the bud-producing leaf being derived like 

 all other leaves from a stem. Epiphyllous buds are even produced by leaves 

 severed fi"om the axis; indeed, in many instances, the severance of the leaves is 

 itself the apparent cause of the development of the buds. This phenomenon is 

 exhibited, for example, by BryophylluTn calicinum, a plant of the House-leek 

 family which belongs to the tropical parts of the Old World, but has long been 

 cultivated in our greenhouses and has attained a certain celebrity even in non- 

 scientific circles, owing to the fact that Goethe interested himself in it and 

 mentions it repeatedly in his writings. The foliage-leaves of this BryophylluTn 

 (see fig. 200') are deeply divided, the separate lobes being oblong-obovate and 

 conspicuously notched. Every full-sized leaf exhibits in each notch of the margin 

 a group of cells, which is perceptible as a dot to the naked eye. So long as 

 the leaf remains upon the stem there is usually no further development of these 

 cell-aggregates, but if the leaf is plucked oft' and laid on the earth an active 

 process of division is set up in them, the result of which is the formation of a 

 little plant with stem, leaves, and roots, as is represented in the figure opposite. 

 The leaves of Bryophyllum calicinum are thick and fleshy, and contain when 

 mature such an abundance of reserve material and water as to render it super- 

 fluous that any absorption of nutriment from the environment should take place. 

 It is not till later that the little plants which spring from the notches of the 

 leaf, having used up the materials stored in the latter, are driven to seek food 

 from the environment by means of their roots. If the leaf has been laid on 

 moderately damp earth, the rootlets of the young plants, developed in its notches, 

 penetrate the ground and, in the event of the tissue of the leaf being in the 

 meantime exhausted and withered, all the little plants become independent and 

 develop into full-sized individuals. Phenomena similar to those exhibited by 

 Bryophyllum calicinum are also observed in other plants with thick, fleshy leaves, 

 particularly in Echeverias. Young plants also make their appearance sometimes 

 on the fleshy leaves of Rochea falcata after they have been picked. There is, it 

 is true, the noteworthy difference that the phenomenon is not foreshadowed, as 

 in Bryophyllum, by the existence of special groups of cells at the points of 

 origin; but Bryophyllum,, Echeveria, and Rochea liave this in common, that in 

 all cases the need of materials for the construction of the young plants is met 



