572 SECONDARV CHANGES. 



vessels. This diflerence between the superseded and the projecting bands of wood 

 is absent in other species. The genus Clytostoma is, according to Bureau, distin- 

 guished from most others by very dense compact wood, with ver}^ narrow vessels ; 

 also, the cortex is generally of normal structure. The bast consists of normal soft- 

 bast, which is traversed by narrow concentric continuous zones of bast-fibres. In 

 the bast-plates the first-formed secondary zones of soft bast are specially narrow, 

 and but little thicker than the zones of fibres alternating with them ; those which are 

 formed later are often on the average of much larger, but alternately variable, radial 

 diameter. The soft bast consists, especially in the plates, of sieve-tubes, mentioned 

 in Chap. V, which are usually wide, and are surrounded in the usual way by delicate 

 cambiform cells. Here also there are various special differences according to genera 

 and species. 



The phenomena hitherto described are uniform for all climbing Bignoniaceae 

 with four intruding plates of bast. Exceptionally, and as an individual peculiarity, 

 the number five is found instead of four. As regards further conditions, ther6 are 

 variations which are in exact correlation with the generic differences founded on the 

 formation of flowers and fruit. 



The number of the superseded bands of wood and intruding bast-plates in a 

 number of genera remains always limited to four as described. In numerous others, 

 on the other hand, in addition to the four primary plates there appear successively 

 new ones in addition as the growth in thickness proceeds : these from their origin 

 onwards behave in all points like the similar primary ones, and in ordinary regular 

 cases are so arranged that each bast-plate of next higher order divides the protrusion 

 of wood, on which it appears, into two almost equal lobes (Fig. 226). All bast-plates 

 of the same rank arise almost simultaneously, and therefore intrude an almost equal 

 distance inwards, the lobes of the wood, successively increased from 4 to 8, 16, 32, 

 therefore show a regular dichotomous division and arrangement. When the stem 

 has grown very old, these conditions may become less regular. 



In many genera each single bast-plate has exclusively the phenomena of growth 

 above described, and therefore always remains equally broad throughout. In others, 

 however, the plates become gradually broader outwards in the following manner : 

 opposite each protmding mass of xylem, after it has grown in thickness to a certain 

 extent, the radial band limiting the bast-plate itself hangs back in its growth, while 

 the portion of cambium adjoining it forms a narrow bast-plate in the manner above 

 described, and is joined laterally on to that first present. Since the same process is 

 repeated periodically — perhaps with each annual layer — as the growth in thickness 

 proceeds, each bast-plate becomes broader outwards, by step-like increments on 

 both sides. Each step-like portion of it, as well as the superseded radial band of 

 wood belonging to it, has the properties above described for the primary ones. 

 According to the individual case, each successive order of steps arises at the two 

 sides of one plate, and of all the plates of one transverse section of a stem at a more 

 or less equal distance from the middle; the whole arrangement of steps is therefore 

 to a variable extent regular or irregular, Comp. Figs. 225, 226. 



The widening of the bast-plates outwards, as described, is continuous, if the 

 segments of wood, which adjoin the successive steps, widen in their further increase 

 in such a way that the limiting faces remain exactly radial (Fig. 225). But in 



