ANOMALOUS THICKENING IN DICOTVLEDONS AND GYMNOSPERMS. 



579 



thicker. But later a change takes place : a dry piece of a stem or branch of S. nux 

 vomica of iss'""* diameter, which was investigated, had a bast zone about 5«ini thick, 

 consisting of very many layers. Climbing species, such as S. toxifera, S. brachiata, 

 &c., seem to form secondary bast in still larger quantities. The secondary layer 

 of bast has, except in one essential point, the same structure, and shows the same 

 phenomena of dilatation as in the Dicotyledons with normal growth. Especially in 

 S. nux vomica, it consists of broad, uniformly delicate, parenchymatous medullary 

 rays, without stony elements, and between these run narrow bands, corresponding 

 to the strands of xylem, each consisting of a few longitudinal rows of elongated cells 

 with oblique or horizontal ends, soft, rather thick, lateral walls, with simple scattered 

 pits, and delicate transverse walls. The strands are accompanied by numerous 

 chambered sacs with crystals. Both bast fibres and sieve-tubes are entirely absent. 

 Tke sieve-tubes are on the other hand situated in the wood. This has in the main the 

 normal structure of Dicotyledonous wood. It consists in the species in question of 

 (i) broad, numerous, medullary rays, com- 

 posed of procumbent cells; (2) narrow 

 strands, and portions of strands of different 

 grades, which are composed of irregularly 

 alternating broad transverse zones of very 

 thick and long wood-fibres on the one hand, 

 and large-celled fascicular parenchyma with 

 pitted vessels on the other. In the mass 

 of wood, which by reason of this com- 

 position appears, in a slightly magnified 

 transverse section, to be marked with ir- 

 regular bands, there lie numerous strands, 

 on an average about 0-30'"™ thick, with a 

 roundish, or broadly elliptical transverse 

 section. They are scattered through the 

 whole annual ring, being usually isolated 

 between two medullary rays, and rather 

 broader than the section of wood which 

 bears them, so that they intrude on both 

 sides into the adjoining medullary rays; 

 sometimes they also pass transversely 

 through 2-3 strands of wood, together 



with the intervening medullary rays. Their longitudinal course follows in the 

 main that of the xylem-strands, with this difference, that they show fewer acute 

 anastomoses than these. These strands are composed of longitudinal rows of 

 thin-walled colourless cells — and their circumference seems to be covered exclusively 

 by these, — and numerous large sieve-tubes, with oblique endings of the joints, bearing 

 sieve-plates arranged in a ladder-like manner. Suitable stages of development show 

 easily that these phloem-strands in the xylem are derived internally from the cambium 

 (comp. Fig. 229). Other species investigated show similar, sometimes even larger 

 phloem-strands in the xylem, and, as far as may be decided from dry material, the 

 same absence of sieve-tubes in the bast. All investigated species have, at the limit 



p p 2 



Fig. 229. — Strychnos, unnamed East Indian species. 

 Portion of a transverse section through a branch {20) ; ?« pith, 

 c cainbial zone, between the two is the xylem with numerous 

 large vessels ; si bundles of sieve-tubes at the limit of the 

 pith; J- the same in the xylem, the two outermost at the 

 Uniit of the cambial-zone, just passing off from it. 



