CIRCULATION AND FORMATION OF WOOD IN PLANTS. 589 



the view of separating its middle layer containing the vessels, I came 

 upon some structures that were new to me. These structures, where 

 they are present, form the terminations of the vascular system. They 

 are masses of irregular and imperfectly united fibrous cells, such as 

 those out of which vessels are developed ; and they are sometimes 

 slender, sometimes bulky — usually,however, being more or less club- 

 shaped. In transverse sections of leaves their distinctive characters 

 are not shown : they are taken for the smaller veins. It is only by 

 carefully slicing away the surface of a leaf until we come down 

 to that part which contains them, that we get any idea of their 

 nature. Fig. 1 represents a specimen taken from a leaf of Eu- 

 phorbia neriifolia. Occupying one of the interspaces of the ulti- 

 mate venous network, it consists of a spirally-lined duct or set of 

 ducts, which connects with the neighbouring vein a cluster of half- 

 reticulated, half -scalarif orin cells. These cells have projections,many 

 of them tapering, that insert themselves into the adjacent intercell- 

 ular spaces, thus producing an extensive surface of contact between 

 the organ and the imbedding tissues. A further trait is, that the en- 

 sheathing prosenchyma is either but little developed or wholly ab- 

 sent ; and consequently this expanded vascular structure, especially 

 at its end, comes immediately in contact with the tissues concerned 

 in assimilation. The leaf of Euphorbia neriifolia is a very fleshy 

 one ; and in it these organs are distributed through a compact, 

 though watery, cellular mass. But in any leaf of the ordinary type 

 which possesses them, they lie in the networkparenchyma composing 

 its lower layer ; and wherever they occur in this layer its cells unite 

 to enclose them. This arrangement is shown in fig. 2, representing 

 a sample from the Caoutchouc-leaf, as seen with the upper part of 

 its envelope removed ; and it is shown still more clearly in a sample 

 from the leaf of Panax Lessonii, fig. 3. Figures 4 and 5 represent, 

 without their sheaths, other such organs from the leaves of Panax 

 Lessonii and Clusia flava. Some relation seems to exist between 

 their forms and the thicknesses of the layers in which they lie. 

 Certain very thick leaves, such as those of Clusia flava, have them 

 less abundantly distributed than is usual, but more massive. Where 

 the parenchyma is developed not to so great an extreme, though 

 still largely, as in the leaves of Holly, Aucuba, Camellia, they are 

 not so bulky ; and in thinner leaves, like those of Privet, Elder, 

 &c, they become longer and less conspicuously club-shaped. Some 

 adaptations to their respective positions seem implied by these modi- 

 fications ; 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 diagrams. 

 Relations of this kind are not regular, however. In various other 

 genera, members of which I have examined, as Rhus, Viburnum, 

 Griselinia, Brexia, Botryodendron, Pereskia, the variations in the 



