154 Mr Seivard, Notes on the Binney Collection 



conform to the structural characters of ordinary phloem. The 

 large clear spaces or sacs which form so prominent a feature in 

 this region cannot, I believe, be satisfactorily explained as the 

 result of decay previous to mineralisation ; their appearance is 

 strongly suggestive of sacs or spaces formed, for the most part, 

 during the life of the plant by the separation and partial disorgan- 

 isation of thin-walled cells. The constant occurrence of patches 

 of a dark brown substance in this zone also points to the secretory 

 nature of the tissue. Before considering the possible function of 

 the secretory zone, there are a few points to be emphasised in the 

 structure of the leaf-traces. The xylem of the leaf-trace is of 

 mesarch structure 1 ; as each trace passes out from the corona, the 

 tracheal strand is accompanied by a few cell-rows of the meriste- 

 matic band {a, a PI. III. figs. 3 and 4), and groups of elements 

 from the secretory zone (s.s, PI. ill. figs. 3 and 4) also form part of 

 the leaf-trace tissues. The group of secretory tissue accompanying 

 the xylem of a leaf-trace (PL iv. fig. 11) may be followed, in 

 longitudinal and in transverse sections, into direct continuity with 

 the broad secretory zone of the stem ; but the elements of this tissue 

 which form part of a leaf-trace differ from the secretory zone in 

 the narrower diameter of the cells and in the somewhat greater 

 abundance of secreted substance. Neither in the structure of the 

 main stele of the stem nor in the tissues of the leaf-trace do we 

 find any set of elements, which exhibit histological features afford- 

 ing satisfactory evidence of the existence of either hard or soft 

 phloem of the ordinary type. 



The apparent absence of any well-defined phloem tissue is a 

 fact of considerable interest ; even in the large Dalmeny stem, in 

 which the secondary xylem is 2 '8 cm. in diameter 2 , there is no 

 indication of any tissue which can be identified anatomically with 

 true phloem. ,* Indeed in no stem of Lepidodendron, Lepidophloios 

 or Sigillaria has typical phloem been so far satisfactorily demon- 

 strated 3 . The existence of the secretory zone suggests a phy- 

 siological comparison of this tissue with normal phloem, on the 

 ground that in certain recent plants laticiferous tissue has been 

 considered as in part at least carrying out the functions of 

 phloem 4 ; it is a probable view that this well-marked zone may 

 have served the same purpose as the tissue which in recent plants 

 usually presents the structural characters of phloem. In position 

 the secretory zone corresponds to the pericycle, and this region of 

 the stem in certain recent species is not infrequently characterised 

 by the presence of numerous laticiferous tubes. Without discuss- 



i Cf. Maslen (99), p. 364. PI. xxxvi. fig. 12. 



2 For a figure of the stele of this stem vide Seward (98), p. 82. 



3 Hovelacque (92) describes phloem in Lepidodendron vasculare (L.selaginoides), 

 but the features it presents do not appear to be those of typical phloem. 



4 Haberlandt (96), p. 295; de Bary (84), p. 187; Tschirch (89), p. 526. 



