ABSORBING SCALES 



241 



located mainly on the leaf-bases, which collectively form a water- 

 reservoir ; in the matted and long-stemmed species (e.g. Tillandsia), on 

 the other hand, both leaves and stems are covered all over with peltate 

 scales. 



The histological structure of such an absorbing scale is in general 

 as follows (Fig. 93). A more or less expanded one- to several-celled 

 basal portion, or foot, is continued into a funnel-shaped stalk, which is 

 sunk below the level of the epidermis. This stalk consists of three 

 or four fiat, thin-walled cells containing abundant protoplasm, which 



Fio. 93. 



Water-absorbing scales of Vriesea psittacina. A. Surface view of the disc of a 

 scale. B. Vertical section of a scale in the turgescent condition. C. Vertical section 

 of a scale in tbe collapsed state. 



represent the actual absorbing apparatus ; it is surmounted by an 

 approximately circular, or, in some cases, irregularly-shaped disc. The 

 marginal cells of the disc are often radially elongated ; in the genus 

 Tillandsia the membrane of these cells is produced into a scarious 

 radially folded wing. In contrast to the stalk -cells, both the marginal 

 and the central cells of the disc are devoid of living contents. The upper 

 (i.e. outer) cell-walls of the disc are only slightly thickened in the 

 case of species which grow in moist shady situations, or which have their 

 scales completely sunk below the surface of the epidermis ; where, on 

 the other hand, the scales project, the corresponding walls are of very 

 considerable thickness. In all circumstances, however, these outer 

 walls of the disc are entirely free from cutin, and are completely 



Q 



