57^ SECONDARF CHANGES. 



This phenomenon occurs in Stryclinos, in all investigated species of the genus, 

 both in those which climb and grow with tendrils — S, colubrina, toxifera, multiflora, 

 and that not exactly defined form figured by F. IMiiller ' — and also in the tree- and 

 shrub-like species — S. nux vomica, brachiata, and innocua. It appears further, though 

 in a ditTerent form to that in Strychnos, in that member of the Malpighiaceae 

 mentioned by F. Miiller'^ as Dicella spec. 



The above-named species of the genus Strychnos were investigated in dry 

 material ; for the investigation of the youngest stages of development a living 

 specimen was used of the plant, which comes to the shops as S. nux vomica, a 

 plant which really belongs to another species. 



The species of Strychnos have a normally arranged ring of bicollateral bundles 

 of the trace in the young internode. Their externally directed (external) phloem, in 

 which, on the first differentiation of the bundles, the first developed elements (Proto- 

 phloem) may be recognised, consists in the fully elongated internode of small groups 

 of about 4-6 narrow elements, which are very similar in transverse section to small 

 groups of sieve-tubes. But I will not say definitely whether they really contain 

 developed sieve-tubes. After those primitive phloem-elements the first vessels appear 

 at the inner margin of the xylem, and almost simultaneously with them the phloem- 

 groups of the inner margin begin to become plain. The further development of the 

 xylem then proceeds in the manner normal for Dicotyledons : in the inner phloem- 

 groups the increase and growth of the elements continues for a long time, and they 

 attain a great size. Numerous intermediate bundles, separated from one another 

 by medullary rays one layer of cells thick, then connect the bundles of the trace into 

 a dense ring. The intermediate bundles contain also, at least in part, the small ex- 

 ternal phloem-groups, but no internal ones: at least they are certainly absent in most 

 cases. An increase of the elements of the small external phloem, in the same way 

 as in the typical Dicotyledons it keeps pace with the growth of the xylem, does not 

 occur in Strychnos ; but rather that layer of cells immediately adjoining those small 

 external phloem-groups, or at least the next inner one, passing round the stem, 

 becomes the mother-layer of the cambium. The origin of this, as could be proved 

 with the small quantity of material available, is fundamentally the same as in normal 

 Dicotyledons ; the same is the case with the direction of the divisions which pro- 

 duce the secondary elements. In' the succession of these, however, and in the de- 

 velopment of their products, the peculiarity occurs that they at first proceed almost 

 exclusively in a centrifugal direction : it is exclusively or almost exclusively secondary 

 v.-ood that is developed. In a one-year-old shoot, \^^^ thick, of the living plant, 

 the xylem-ring of which showed on the radius of a transverse section 10-12 elements, 

 and among them wide vessels, there lay in each radial row, between the outermost 

 mature xylem elements and the small external phloem-groups, only two cells, 

 separated by one tangential wall, or even only one thin-walled cell. These belong to the 

 cambium and zone of increase, the latter is thus directly contiguous with the primary 

 zone of bast : it forms with this a narrow ring round the xylem, limited externally by 

 a dense la) er of short sclerenchyma. This condition remains for a long time ; in a 

 dry branch of S. nux vomica 2-^^^^ thick I find the bast layer not at all or but little 



'^ I.e. Fig. 10. ° I.e. p. 59. 



