176 



LECTURE XI. 



substances in laticiferous vessels, in the narrower sense, it cannot 3-et be concluded 

 whether a sharp distinction exists between segmented laticiferous vessels and 

 serially arranged secretion-vesicles. Very widely distributed are, further, certain 

 tannins, often mixed with a red colouring matter. These are found in special 

 individuaHsed cells or in rows of cells, and are not again employed in metabolism, 

 and are therefore to be considered as excreta; whereas in other cases {e.g. the 

 plumule of the Oak) tannins of another kind are to be recognised, from their 

 origin and disappearance, and by their behaviour in growth, as special forms of 

 reserve-materials, which find further employment in metabolism, and are therefore 

 to be excluded from our present considerations. 



It would, however, be in vain to attempt a thorough classification of the 

 organs of secretion according to the chemical nature of their excreta; and 

 a purely histological classification would lead just as little to a satisfactory 

 result. We will confine ourselves in what follows, therefore, chiefly to the 

 general characters of the organs concerned, without attempting a rigid sifting 

 according to this or that point of view. It may be mentioned in anticipation, 

 however, that the receptacles for secretion^ present anatomically the greatest 

 possible variety. Very often they are individual cells, lying in the tissues, which 

 contain calcium oxalate, mucus, resin, tannin, etc. : or such cells are arranged 

 in long rows, which then usually follow the vascular bundles, or lie in the soft 

 bast. Or the receptacles for secretion are intercellular spaces, which become filled 

 with the secretion from the cells bounding them; and these intercellular spaces 

 may either be long, more or less narrow canals, or form roundish or elongated 

 sacs. Or serially arranged or roundish cell groups become disorganised, and 

 the cavity thereby arising (according to De Bary a lysigenous intercellular space) 

 remains filled with the products of decomposition of the cells. Finally, secretions 

 may arise in the wall separating neighbouring cells; or even between the cuticle 

 and the proper cell wall in the epidermis and hair-like structures, as is often the 

 case with the numerous epidermal glands in the Phanerogams. 



The fact that the receptacles of secretions, as well as the laticiferous vessels, 

 with rare exceptions, make their appearance in the very youngest state of the 

 organs, at the beginning of the differentiation of their embryonal tissues, throws 

 light on their physiological signification. They are already sketched out or deve- 

 loped when the tissues of the vascular bundles and the fundamental tissue first 

 commence to obtain their characteristic structure. Apparently the developing tissue 

 rids itself during nutrition of certain products of decomposition even thus early; and 

 these remain lying in the receptacles of secretion as such without further use. 



Here also the higher degree of organisation of the shoot, in comparison with the 

 root, makes itself evident, in that receptacles of secretions, which appear in the shoot- 

 axes and leaves, are not rarely wanting to the roots, or are more feebly developed in 

 them ; whereas they usually appear very abundantly in the flowering shoots of the 

 Phanerogams. Families which possess no receptacles of secretions at all are 

 relatively rare among the Phanerogams — e.g. the Gravmiece and Cyperacece, 



^ De Bary's description (' Vcrgl. Anat' Cap. Ill) of the receptacles for secretions is the only 

 satisfactory one hitherto. 



