UNICELLULAR HYDATHODES 489 



cutinised. The third and innermost portion of the hydathode, finally, 

 consists of a thin-walled chamber, which is sharply delimited from 

 the thick-walled funnel. It is highly probable that this chamber 

 regulates the volume of the whole hydathode in accordance with the 

 large alterations of pressure which doubtless occur under varying con- 

 ditions. The hydathodes of Gonocaryum contain a well-developed 

 protoplast and a large nucleus, features which are typical of glandular 

 cells in general. On an average, there are 58 of these water-secreting 

 organs per sq. mm. of surface on the lower side of the leaf ; the 

 corresponding number for the upper side is 55. 



No less remarkable is the structure of the unicellular hydathodes 

 which occur on both sides of the leaves of Anamirta Cocculus (Menis- 

 permaceae) (Fig. 193 c). Each lies in a shallow depression of the 

 surface, and resembles an inverted funnel in shape. The inner and 

 lateral walls, though unthickened, are strongly lignified. To the middle 

 of the thickened outer wall is attached a peculiar " filtering apparatus," 

 composed of a small external papilla, and a long peg-shaped internal 

 process, which usually terminates in a number of knobs. The peg is 

 traversed by a longitudinal canal, which is narrow for the greater part 

 of its length, but expands to a varying extent in the papilla. At the 

 tip of the papilla the cuticle is absent, or at any rate (Fig. 193 e) 

 perforated by numerous pores. The cellulose layers in the papilla are 

 modified in a mucilaginous manner. This part of the cell, therefore, 

 really represents a funnel filled with mucilage, open above and con- 

 tinuous below with the narrow lumen of the peg. The outer layers in 

 the wall of the peg are all strongly lignified ; a delicate striation which 

 often appears in this part of the structure, is due to the presence of 

 numerous narrow, transversely elongated pits. In this case, also, the 

 hydathode contains a massive protoplast and a large nucleus. According 

 to Krafft, very similar water-secreting organs occur in the allied genus 

 Arcangclisia, 



From such unicellular hydathodes the transition is easy to the multi- 

 cellular "water-glands" of the Plumraginaceae, the structure and 

 action of which have been studied in detail by Volkens. Here the 

 approximately hemispherical gland consists, as a rule, of four central 

 and four peripheral cells. All these eight cells have abundant proto- 

 plasmic contents and unthickened walls. Even the outer walls are 

 thin. Only those partitions which separate the gland from the sur- 

 rounding tissues are somewhat thickened, besides being highly refractive 

 and insoluble in sulphuric acid* Each gland arises by means of appro- 

 priate divisions from a single protodermal mother cell ; it is usually 

 enclosed by four epidermal subsidiary cells, which appear crescent- 

 shaped in section (Fig. 194). 



