Recueil des travaux botaniques néerlandais . ne, two or three. Thisis the common number. This is considerably less than themaximum found m B. lunaria, where Poirault (25) countedseven proto-xylems. The vascular bundle in a diarch root couldbe traced in a continuons séries of sections. In the youngestpart, the xylem is pronouncedly exarch and consists of severalreticular or spiral tracheids, before either endodermis or phloemappear. At an older stage (Fig. 29) the two proto-phloems 359 appear, thick-walled cells, alternating with the xylems. Distinctsieve tubes, even in the older stages, could
Recueil des travaux botaniques néerlandais . ne, two or three. Thisis the common number. This is considerably less than themaximum found m B. lunaria, where Poirault (25) countedseven proto-xylems. The vascular bundle in a diarch root couldbe traced in a continuons séries of sections. In the youngestpart, the xylem is pronouncedly exarch and consists of severalreticular or spiral tracheids, before either endodermis or phloemappear. At an older stage (Fig. 29) the two proto-phloems 359 appear, thick-walled cells, alternating with the xylems. Distinctsieve tubes, even in the older stages, could not be traced. Atthis stage the endodermls beglns to develop. The pericycle Isabsent between proto-phloem and endodermis. Between xylemand endodermis there is a pericycle consisting of one layer,the cells of which are filled with dense protoplasm. Still laterthe two proto phloems join (Fig, 30) to form a linear strand ofxylem between the two phloems. The xylem increases in bulkbefore the leaf trace joins it. The point of their junction can. Fig. 40. be seen in Fig. 31. The endodermis, previously broken bythe entering leaf-gap, surrounds the stele again entirely. Thephloem follows the inside of the endodermis, leaving onlyan occasional pericycle cell between. Bower (4) in his work on the medullation of the Ophioglossa-ceae remarks that the pith is initiated below the departure ofthe first leaf trace; but that the medullation is is no évidence that the medullation in is anything else than the enclosure of a cer-tain number of parenchyma cells between twoxylem strands. 360 The internai endodermis, as observed by Poirault (25)and other authors, in B. lunaria is entirely absent in B. behaviour of the endodermis in the latter species is muchsimpler than that found by Lan g for B. lunaria (22). The endo-dermis is absent in the leaf, but surrounds the stem and rootstèle with a complète ring, only interrupted by the entranceof other roots. F
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