492 



SECRETORY AND EXCRETORY SYSTEMS 



component of the whole apparatus. Its lateral walls are often greatly 

 thickened or at any rate strengthened by means of a stout flange, and 

 are, in addition, nearly always strongly cutinised. As a consequence 

 this cell acts as a rigid annular frame which prevents the " internal 

 water-pore" (i.e. the aperture through which water passes from the 

 general mesophyll into the actual secretory organ) from altering its 

 dimensions. The unicellular or multicellular base, finally, which is often 

 more or less distended, is responsible for all communication between 



the rest of the hydathode and the adjoining 

 epidermal and sub-epidermal cells. It is, 

 accordingly, always thin-walled ; its form, 

 moreover, is often such as to bring the 

 greatest possible number of palisade-cells, or 

 other sub-epidermal elements, into actual 

 contact with the base of the hydathode. 



The epidermal hydathodes which occur in 

 the cavities of the scale-leaves of Lathraea 

 squamaria, require separate consideration. The 

 secretion of water by the scale-leaves of this 

 plant was first observed by Charles Darwin, 

 and has since been studied in detail by the 

 author, by Goebel and by Groom. The 

 interior of every scale-leaf of Lathraea is 

 occupied by a continuous system of channels 

 and clefts, which opens to the exterior through 

 a single narrow outlet. The wall of this 

 cavity corresponds morphologically to the 

 abaxial surface of the scale-leaf ; it bears 

 glandular structures of two distinct types, 

 which may respectively be termed capitate and scutate glands. Each 

 capitate gland is composed of a one- or two-celled base, a stalk-cell, 

 and two head-cells with abundant protoplasmic contents (Fig. 197). 



The scutate glands (Fig. 196) have been studied in detail by 

 Scherffel. Each of these consists, first, of a large lenticular cell, the 

 bulk of which is sunk below the surface of the epidermis ; and, 

 secondly, of a tier of four cells superimposed upon the basal cell and 

 collectively forming a lenticular or concavo-convex structure. The two 

 middle cells of the group are not completely joined together, but 

 enclose a groove which is roofed over by the cuticle. This cuticular 

 film is perforated, at the exact apex of the whole gland, by a minute 

 circular pore, not more than "004 mm. in diameter, through which the 

 aforesaid groove communicates directly with the exterior. The large 

 lenticular basal cell rests upon a rosette-shaped group of eight cells, 



Fig. 196. 



Scutate glands from a scale- 

 leaf of Lathraea squamaria. A. 

 Surface view. B. Vertical sec- 

 tion ; the small rod-shaped bodies 

 adhering to the gland are bac- 

 teria. 



