£94 Scott and Brebner . — On the Anatomy 
question which has been sufficiently dealt with by Petersen 
and Solereder. In several of these cases the normal ex- 
ternal phloem is much reduced, but this peculiarity is more 
generally characteristic of plants with phloem-islands in the 
wood. In Strychnos , which is so abundantly provided with 
both medullary and interxylary phloem, it can excite no 
surprise that the external phloem should remaih in a com- 
paratively rudimentary state. The presence of a centrifugal 
cambium between the xylem and the internal phloem of 
bicollateral bundles has already been often recognized. 
Vesque 1 found it in various Solanaceae, Asclepiadeae, and 
Apocyneae, and Petersen further showed that it occurs among 
the Myrtaceae, Lythraceae, Cichoriaceae and Campanulaceae 2 . 
He even states that in Tragopogon and Lactuca this cambium 
forms a little centripetal wood. The internal cambium of 
Strychnos, especially of 5. nux vomica , appears to be excep- 
tionally active in phloem-formation, so much so that a 
comparison with Tecoma at first suggested itself. There is, 
however, no centripetal wood in Strychnos , and the medullary 
structures in Tecoma must certainly be regarded as consti- 
tuting a distinct system of bundles, whether cauline as Herail 3 
and Hovelacque 4 believe, or common, as held by Weiss 5 , and 
not as forming part of the bundles of the normal ring. How- 
ever, the distinction is not really so sharp as it appears. 
Transitional cases between internal phloem and distinct 
medullary bundles occur in the Melastomaceae, Solanaceae, 
Cichoriaceae, Campanulaceae 6 , and other families. 
As regards the presence of phloem-islands in the wood, 
they are now known, chiefly owing to Solereder’s work on her- 
barium material, to occur in no less than twenty-four genera, 
belonging to ten different natural orders ; whereas when 
de Bary published his ‘ Vergleichende Anatomie ’ they were 
only known in the two genera Strychnos and Dicella. We will 
not enumerate all the genera, but will only mention the orders 
1 1. c. p. 146. 2 1. c. pp. 363 (PI. IV, Fig 4), 370, 386, and 391. 
3 1. c. p. 282. 4 1. c. pp. 12, and 146-179. 5 1. c. pp. 396, etc. 
6 Cf. Petersen, 1. c. pp. 371, 382, 386, 391 ; also Weiss, 1. c. p. 396. 
