520 SECRETORY AND EXCRETORY SYSTEMS 



limit of elasticity. These cells must therefore contain large quantities 

 of osmotically active material. The high pressure to which the con- 

 tents of the gland are thus exposed, is nevertheless not in itself 

 sufficient to bring about the formation of the gland-pore and the 

 ejection of the contents. The latter never become spontaneously dis- 

 charged. On the contrary, the secretion only escapes when the leaf is 

 forcibly bent, so that the pressure upon the glands is suddenly increased ; 

 the tension set up in the cover-cells on the convex side of the bent leaf 

 assists in the formation of the gland-pore. Vigorous shaking suffices 

 to firing about the discharge in the case of very turgid shoots of Ruta 

 gravcolcns. 



The author's observations on this point have been extended by 

 Detto, who has shown that the shortly-stalked spherical or pearl-shaped 

 glands on the flowering shoots of Dictamnus are also furnished with a 

 special discharging mechanism, consisting essentially of a multicellular, 

 hair-like beak attached to the apex of the gland proper ; at the 

 slightest touch, the beak breaks off and the contents of the gland are 

 ejected. Detto's figures suggest that the lowermost transverse walls in 

 the beak act as " pore- walls." 



It is not only among the Eutaceae that internal glands are 

 generally provided with a discharging apparatus ; a similar device also 

 occurs in all the Myrtaceae that have been examined from this point 

 of view, though its construction is not quite the same as in the first- 

 mentioned family. The author has given a brief account elsewhere of 

 the discharging arrangements of Myrtus communis, while Porsch has 

 made a detailed study of the corresponding structures in Eucalyptus 

 globulus and E. pulverulenta. Here the " cover " consists typically of 

 a single pair of cells ; in Eucalyptus both the outer and the inner walls 

 of the cover-cells are very thin. The septum between these two cells 

 is curved in an S-shaped manner, and is greatly thickened and 

 irregularly pitted. The gland-pore again comes into being when the 

 leaf is forcibly bent ; in this case, however, it arises, not by fission of 

 the septum, but by rupture of the thin outer and inner walls of one 

 or both cover-cells. Otherwise the discharging mechanism is the same 

 as in the Eutaceae. In the present instance, the functions of the 

 highly turgescent body-cells are vested in the persistent secretory cells, 

 which themselves exert a very considerable pressure upon the contents 

 of the gland. According to Porsch, the internal glands which occur in 

 the petals of Boronia megastigma are provided with a discharging 

 mechanism of the Ruta type. 



It has already been stated, that internal glands are frequently 

 enclosed in a special sheathing layer. The functions of this sheath are 

 not always the same. In the case of a lysigenous gland it serves prin- 



