178 T, Holm — Dirca Palustris. 



geneous, thin-walled cork of several strata surrounds a 

 broad zone of secondary, collenchymatic cortex, of which 

 the peripheral two to three layers contain deposits of 

 starch; in the cortex are many strands of stereids, mth 

 the lumen narrow, and the cell-walls neither porous nor 

 lignified (fig. 3. St.). Narrow rays of starch-bearing 

 parenchyma extend from the stele through the cortex to 

 the cork. The stele represents a continuous, broad zone 

 of leptome, interspersed with scattered strands of stere- 

 ome or single stereids (figs. 3-4). Inside the leptome is a 

 well developed cambium, a large mass of radially 

 arranged libriform (S in fig. 3) and some isolated strands 

 of vessels ; the cells of the libriform are, however, rather 

 short, but porous, and with the cross-walls oblique. The 

 center of the stele is occupied by a few narrow, spiral ves- 

 sels, and there is no pith. 



With respect to the primary structure of the root, none 

 of the roots of my material were sufficiently young for 

 studying this state, but Van Tieghem^ states, that in Dirca 

 the young root is triarch, and that the leptome contains 

 numerous stereids, which are strongly lignified, and that 

 several of these border directly on the pericambium. 

 Moreover in describing the root-structure in general of 

 the ThymelaececB this author calls attention to the fact, 

 that the exodermis shows the same degree of thickening 

 as the endodermis, but while in the former (exodermis) 

 the peripheral cell-wall is thickened and the interior thin- 

 walled, the opposite is the case of the endodermis. 

 According to Van Tieghem (1. c.) the presence of stereids 

 in the leptome is a very rare occurrence, known so far 

 only from Anona, CeltiSy Leguminosce and Malvacece. 



'Eecherehes sur la structure et les afSLnites des Thymeleacees et des 

 Peneacees, Ann. d. sc. nat. Bot. Ser. 7, vol. 17, Paris, 1893. 



Pig. 1. — Flower of Dirca occidentalis, laid open, showing four of the 

 stamens ; enlarged. 



Fig. 2. — Flower of Dirca palustris, laid open; enlarged. 



Fig. 3. — Cross-section of a thick, lateral root, showing the stereids (St.) 

 ia the leptome (L.) ; camb = cambium ; S = libriform; V = vessel. X 496. 



Fig. 4. — Cross-section of part of leptome of same root, showing the 

 stereids X 600. 



Fig. 5. — Cross-section of inner part of stem, showing cambium inside the 

 hadrome; S-z= spiral vessels. X 496. 



Fig. 6. — Superficial section of dorsal face of leaf showing the stomata. 

 X320. 



Fig. 7. — Cross-section of leaf ; Ep. =: ventral, Ep. * z=z dorsal epidermis ; 

 P. = palisade-tissue ; P * = pneumatic tissue. X 496. 



(Figs. 3-7 are D. palustris.) 



