INTERNAL GLANDS 521 



cipally to shut off the contents of the gland from the surrounding 

 tissues. Among schizogenous glands, on the other hand, the segrega- 

 tion of the contents is effected by the continuous (persistent) secretory 

 layer itself ; here the function of the sheath is probably in the first 

 instance mechanical. The special part played by the sheath in relation 

 to the discharging process in the Eutaceae, has already been explained. 

 The sheathing layers of glands may arise in a variety of ways. Thus, 

 the outer walls of the secretory cells may become thickened where 

 they abut against the surrounding tissues {e.g. in the smaller glands of 

 Myrtus zcylanica, Fig. 210 b). The sheath may, on the other hand, 

 consist of an irregular layer of collapsed cells, derived from the sur- 

 rounding tissues ; these have thickened walls, and their contents do 

 not differ noticeably from those of the neighbouring unmodified 

 elements (e.g. Eugenia australis and Eucalyptus cornuta, according to 

 Von Hohnel). The sheath may, lastly, take the form of a regular and 

 continuous layer of flattened cells, with somewhat thickened walls and 

 colourless contents {Hypericum perforatum). Occasionally, combinations 

 of these modes of development occur {e.g. in Myrtus zeylanica, where 

 the two first-mentioned methods of development are combined in the 

 case of the larger foliar glands). 



Chemically considered, the substances secreted by internal glands 

 are generally in the nature of ethereal oils or resins. What has been 

 stated above concerning the ecological value of dermal glands, applies 

 equally to internal glands, where the secretion is of a similar nature 

 in both cases. The discharging arrangements of the Eutaceae and 

 Myrtaceae probably provide additional protection against the attacks 

 of animal foes ; presumably the animals take fright on suddenly 

 encountering an intense odour which is not noticeable until the 

 plant is roughly handled. An analogous method of protection is, of 

 course, well known to be widely distributed in the animal kingdom. 



E. SECRETORY DUCTS AND PASSAGES (OIL-, RESIN-, GUM- AND 



MUCILAGE-PASSAGES). 253 



Secretory ducts or passages only differ from other internal glands in a 

 single particular ; they are always more or less elongated, and accord- 

 ingly may, like latex-tubes, traverse whole organs, or even more 

 extensive regions of the plant-body, from end to end. That there is 

 no. other difference between the two types of secretory organ, may be 

 shown by the following consideration, among others : In the scale- 

 leaves of Thuja and Biota, the resin-passages which are so characteristic 

 a feature of the needles of Pinus and Abies, are replaced by glands of 

 rounded outline. Structures intermediate between glands and secretory 

 passages are also known. In the case of Ginkgo biloba, for example, it is 



