330 THE MICROSCOPE. 



" Some adaptations to their respective positions seem 

 implied by these modifications ; and we may naturally 

 expect that in many thin leaves these free ends, becoming 

 still narrower, lose the distinctive and suggestive characters 

 possessed by those shown in the drawings. Relations of 

 this kind are not regular, however. In various other 

 genera, members of which 1 have examined, as Rhus, 

 Viburnum, Griselinia, Brexia, Botryodendron, Pereskia, 

 the variations in the bulk and form of these structures are 

 not directly determined by the spaces which the leaves 

 allow; obviously there are other modifying causes. It 

 should be added that while these expanded free extre- 

 mities graduate into tapering free extremities, not differing 

 from ordinary vessels, they also pass insensibly into the 

 ordinary inosculations. Occasionally, along with numerous 

 free endings, there occur loops ; and from such loops there 

 are transitions to the ultimate meshes of the veins. 



" These organs are by no means common to all leaves. 

 In many that afford ample spaces for them they are not to 

 be found. So far as I have observed, they are absent from 

 the thick leaves of plants which form very little wood. 

 In Sempervivum, in Echeveria, in Bryophyllum they do not 

 appear to exist ; and I have been unable to discern them 

 in Kalanchoe rotundifolia, in Kleinia ante-euphorbium, and 

 ficoides, in the several species of Crassula, and in other suc- 

 culent plants. It may be added that they are not absolutely 

 confined to leaves, but occur in stems that have assumed 

 the functions of leaves. At least I have found, in the 

 green parenchyma of Opuntia, organs that are analogous, 

 though much more rudely and irregularly formed. In 

 other parts, too, that have usurped the leaf-function, they 

 occur, as in the phyllodes of the Australian acacias. 

 These have them abundantly developed ; and it is interest- 

 ing to observe that here, where the two vertically-placed 

 surfaces of the flattened-out petiole are equally adapted to 

 the assimilative function, there exist two layers of. these 

 expanded vascular terminations, one applied to the inner 

 surface of each layer of parenchyma. 



"Considering the structures and positions of these organs, 

 as well as the natures of the plants possessing them, may 

 we not form a shrewd suspicion respecting their function 1 



