EPITHEM-HYDATHODES 493 



separated from one another by more or less well-developed inter- 

 cellular spaces. As the study of development shows, this group of 

 cells represents the base or "foot" of the gland; the large middle cell 

 may be held to represent the stalk, while the apical tier constitutes 

 the head, which is the actual secretory apparatus. The entire sub- 

 epidermal layer of the cavity-wall resembles spongy parenchyma in 

 structure ; but its intercellular spaces, instead of being filled with air, 

 contain water which is pumped into them from the tracheides of the 

 bundle-ends that are present in great numbers in the scale (Fig. 107 i). 

 The glands are thus not directly connected with 

 the vascular system, but draw their supply of 

 water from the sub-epidermal system of inter- 

 cellular spaces. 



It is still uncertain, whether either the 

 capitate or the scutate glands are alone re- 

 sponsible for the secretion of water, or whether 

 both organs take part in the process. Both 

 Goebel and Groom regard the scutate glands as 

 the actual hydathodes. 241 



We must now pass on to consider the 

 second type of hydathode, namely, that which is Fl - li,? - 



characterised by the fact that the gland is in JSAtSSlJaS^ 

 direct communication with the water-conducting fSffiSS^SffiitS 

 system. The simplest variety of this second JJJiSff f tracheides 

 type is exemplified by the epidermal hydathodes 



which occur on the leaves of many Ferns {e.g. spp. of Polypodium, 

 Aspidium and Nephrolepis), where they are either uniformly scattered 

 over the surface or else chiefly located in a continuous series along 

 the leaf margin. In this case, each hydathode consists of a group of 

 thin-walled glandular epidermal cells sunk in a shallow depression, 

 immediately above an enlarged bundle-end, which is largely composed 

 of tracheides. An even more widely distributed type of hydathode is 

 that which is provided with so-called water-pores. In a large number 

 of cases, these hydathodes represent modified bundle-ends at the tip 

 or margin of the leaf ; in the Moraceae and Urticaceae, many nodal 

 points of the general vascular reticulum become similarly modified. 

 The bundle-ends in question are usually enlarged to form a more or less 

 club-shaped structure, while the terminal tracheides generally diverge in 

 fan or pencil fashion, and insert themselves between the elements of 

 the epithem, a mass of thin-walled tissue interpolated between the 

 epidermis and the bundle of tracheides. The innermost epithem-cells 

 are often elongated in the same direction as the adjoining tracheides ; 

 all the elements of the epithem offer a marked contrast to the 



