^y6 SECONDARY CHANGES. 



same structure, is continuous externally into the equidistant- protrusions h, which 

 are wedge-shaped in transverse section. With the exception of their inner portion 

 which belongs to the medullary sheath, the intervening bands of wood, which hang 

 back in their growth in thickness, consist, on the other hand, for the most part 

 of thin-walled cells arranged in radial rows, amongst which are scattered isolated 

 pitted vessels, each being accompanied by a few thick-walled elements. The bast 

 plates {b) consist in the main of the large, sharply ended sieve-tubes mentioned 

 in chap. V; these lie very regularly in radial or tangential rows, being usually 

 separated from one another in a radial direction by single, triple, or quadruple 

 concentric series of narrow elements (Fig. 228), rarely they are arranged con- 

 tiguously in pairs ; in a tangential direction, however, they are usually in immediate 

 connection, being rarely separated by the narrow elements (Fig. 228). These narrow 

 elements are for the most part sclerenchymatous fibres, less frequently delicate 

 cambiform cells, of which usually 1-2 abut on each sieve-tube, as seen in transverse 

 section. The peculiar distribution of the thick-walled and other elements causes 



Fig, 227. 



Fig. 228. 



FIG. 227.— Phytocrene spec. Transverse section of tlie stem, X 2. from tlie same material as that described by Met- 

 temus ; h tile eiglit protrusions of tlie porous wood ; * the bast.plates between thcin. Laterally from one of them, and to 

 the right of the wood-protrusion marked h, are two small similar ones. Parenchyma of pith and cortex are left while. Ex- 

 ternally to the inner circle of growth is one of renewed growth, which shows several bundles, between broad parenchy- 

 matous medullary rays, opposite each of the inner protrusions of wood; these consist of a relatively large xylem, con- 

 structed sunUarly to the latter, and an extremely small bast portion i and opposite each of the inner bast-plates is one 

 bundle, oral most two to three, constructed like the latter, which have only avery small xylem at their inner side. The latter 

 could hardly be indicated in the Fig. 



FIG. 228.— Portion of a bast-plate from Fig 227 (X about 140) ; s—s sieve-tubes. The narrow meshes of simple contour 

 abutting on these are transverse sections of the cambiform ceUs ; the elements with a strong double contour are bast fibres. 



the almost chess-board-like appearance of the plates; at the outermost oldest parts of 

 them the arrangement is less regular, the sclerotic elements are in larger proportion. 

 A multiseriate broad medullary ray, consisting of very dehcate cells, limits each lateral 

 face of the bast-plate, and is continuous towards the pith along-side of the superseded 

 band of wood. It appears, as in the Bignonias, to consist in not very young specimens 

 of two radial portions, which undergo displacement. There are at least indica- 

 tions of a step-like external widening of the bast-plates. Externally from the zone 

 of cambium surrounding the protrusions of wood,_that is in the normally arranged 

 layer of bast, there are scattered irregularly, here and there, small irregular groups 

 of the same sort of tissue as that which composes the bast-plates. With the 

 exception of these groups it appears from transverse sections that the bast layer 

 in question contains no sieve-tubes in by far its greater part, but is composed 

 rather of thin-walled parenchyma only, with isolated thin sclerenchymatous fibres. 



