262 Scott and Brebner.—On Internal Phloem 
normal phloem-strands of the root give off branches, some of 
which pass into the pith and are continuous with the internal 
phloem of the stem. He figures the transitional region in 
Datura 1 . Fischer described very fully the transition from 
stem to root in Cucurbita. He shows clearly how the in- 
ternal phloem of the stem passes to the outside of the primary 
xylem and for some distance forms a ring between the latter 
and the secondary wood, gradually disappearing lower down, 
but long remaining traceable in the medullary rays. His 
statement that the transitional phloem-ring between primary 
and secondary xylem ends blindly below, gives, we think, a 
wrong impression. He purposely limited his investigation to 
a much thickened axis, and this fact somewhat impairs the 
clearness of his results 2 . Lastly Dangeard, who has most 
recently investigated the mode of union of root and stem, 
merely states that in the Convolvulaceae the internal phloem 
accompanies the cotyledonary traces as far as their lower 
extremity in the hypocotyl, a statement which throws little 
light on our subject 3 . 
Our most interesting information on the roots of plants 
with bicollateral stem-structure is due to Weiss 4 , who has 
shown that in many of these plants, and especially those 
with fleshy roots, the parenchymatous xylem of the latter 
contains strands of phloem, produced internally by the cam- 
bium. This interxylary phloem in the root was found by 
him in various Onagraceae, Gentianeae and Solanaceae, and 
he considers it a characteristic peculiarity of plants with 
medullary phloem in the stem. 
Our observations extend those of Weiss, and may perhaps 
throw some further light on the relations between the two 
1 R. Gerard, Passage de la racine a la tige; Ann. des Sci. Nat. Bot. Ser. 6, 
T. XI. 1881, pp. 358, 376, PI. 18, Fig. 54. 
2 Fischer, Siebrohrensystem der Cucnrbitaceen ; Berlin, 1884, p. 53, PI. VI. 
Fig. 14. 
3 Dangeard, Recherche sur la mode d’union de la tige et de la racine ; Le 
Botaniste, vol. I. 1889, p. no. 
4 J. E. Weiss, Anat. und Physiol, d. fleischig verdickten Wurzeln ; Flora, 1880 ; 
Markstandiges Gefassbiindelsystem und Blattspuren ; Bot. Centralblatt, vol. 15, 
1883, pp. 397, 409. 
