T. Holm — Dirca Palustris. ISl 



The development of cambium as a closed ring on the 

 inner flank of the hadrome as in Dirca (fig. 5) occurs 

 furthermore in Daphne and Aqidlaria. This cambium 

 may give rise to both leptome and hadrome, inversely 

 arranged, as described by Van Tieghem (1. c.) being 

 characteristic of Aquilaria Agallocha. 



The cork-cambium develops either in the epidermis or 

 in the peripheral stratum of the primary cortex. Very 

 little seems to be known about the cortex, which in Dirca 

 exhibits quite a singular structure ; in Gnidia and Thyme- 

 laea the parenchyma is developed in the shape of a pali- 

 sade-tissue (Van Tieghem, Lc). 



The leaf. — The structure is dorsiventral ; the cuticle is 

 thin and smooth on both faces of the leaf -blade, and the 

 epidermis shows only the outer cell- wall slightly thick- 

 ened. Between the ribs the lumen of epidermis is rela- 

 tively wide on both faces ; in superficial sections the lat- 

 eral cell-walls of epidermis are nearly straight on the ven- 

 tral face, but prominently undulate on the dorsal {fig. 6). 

 The stomata {^g. 6) lack subsidiary cells, and are sunk. 

 Long, unicellular, pointed hairs abound on the dorsal 

 face ; they are slightly thick-walled and covered with cuti- 

 cular pearls. 



The mesophyll consists of a ventral layer of palisade- 

 cells {^g. 7) covering an open pneumatic tissue of three 

 layers ; aggregated crystals abound throughout the mes- 

 ophyll. The midrib is supported by a hypodermal layer of 

 collenchyma passing into a dorsal water-storage tissue of 

 about six layers, containing single and aggregated crys- 

 tals. A single, bicoUateral mestome strand constitutes 

 the midvein, supported by an arch of stereome on both 

 faces; there is no endodermis. 



The very short petiole is hemi-cylindric, and very hairy. 



The epidermis is quite thick-walled, and the cortex is 

 collenchymatic throughout, with chlorphyll in the ven- 

 tral strata. It contains a central mestome strand of the 

 same structure as observed in the midrib of the blade. 



We have thus the bicoUateral structure of the mestome 

 strands represented also in the midvein of the leaf and the 

 petiole although Van Tieghem (1. c.) enumerates Dirca 

 among the genera, where this structure is confined to the 

 stem. It may be that this author had no access to fresh 

 material, since the structure is certainly very conspicuous 



