STRUCTURE OF PLASTTS. 329 



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. 172, ISTo. 1, repre- 

 sents a specimen taken from a leaf of Euphorbia neriifolia. 

 Occupying one of the interspaces of the ultimate 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-scalariform cells. These 

 cells have projections, many of them tapering, which insert 

 themselves into the adjacent intercellular spaces, thus 

 producing an extensive surface of contact between the 

 organ and the embedding tissues. A further trait is, that 

 the ensheathing prosenchyma is either but little developed 

 or wholly absent ; and consequently this expanded vascular 

 structure, especially at its end, comes immediately in con- 

 tact 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 network of 

 parenchyma, composing its lower layer ; and wherever they 

 occur in this layer its cells unite to enclose them. This 

 arrangement is shown in 2\To. 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, No. 4. 

 Nos. 6 and 7 represent, without their sheaths, other such 

 organs from the leaves of Panax Lessonii and Clusia 

 fiava. 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. When 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. 



