LECTURE XL 



LATICIFEROUS VESSELS AND RECEPTACLES FOR SECRETIONS. 



The epidermal tissue, the vascular bundles, and the fundamental tissue 

 may be traced from their most rudimentary beginnings in the Thallophytes, 

 through the Muscine» and Vascular Cryptogams up to the most highly developed 

 Phanerogams as structures phylogenetically identical. In them is expressed an 

 essentially similar plan of organisation of the whole vegetable kingdom ; and even in 

 the product of the cambium of the woody plants we discover nothing essentially 

 new, but only a further development of these tissue-differentiations. 



The fact is quite otherwise with the laticiferous vessels and organs of secretion, 

 which allow no such phylogenetic continuity to be recognised. At the most 

 various stages of development of the vegetable kingdom, we meet with organs 

 of this kind in isolated small subdivisions; while they are wanting in other groups, 

 often very extensive, or appear only here and there in them. In general, we 

 have always to recognise in their existence, however, a sign of further advanced 

 physiological division of laliour ; only the matter is especially one of chemical 

 problems, which are to be solved by means of these relations of organisation. 

 Since the division of physiological labour is most distincdy expressed in the 

 Phanerogams generally; so, too, the forms of tissue here to be considered occur 

 in them more frequently and in greater variety than in the Cryptogams, though 

 they are by no means wanting to the latter. 



It is especially characteristic of the laticiferous vessels and organs of secretion 

 that, while similar in nature in other respects, they are not exclusively peculiar to 

 any one of the three systems of tissue, but are curiously independent in their 

 occurrence. We find laticiferous vessels and organs of secretion sometimes, though 

 more rarely, in the epidermal tissue ; at others in the fundamental tissue, or in the 

 vascular bundles. In this connection the most that can be said is that they })articu- 

 larly affect the fundamental tissue. 



We may first consider the Laticiferous Vessels^. 



In a large number of families, and genera within certain families, or even 

 species within certain genera (e.g. in the Euphorbiaceaj, Urticacese, Asclepiadeas, 

 Papaveraceae, Campanulaceae, Lobeliacese, Cichoriacese, etc.), it is remarkable that from 

 any wound, however small, there exudes at once a thick drop, or c\cn a sti'eam 



' In so far as the description here given differs from that in my ' Tcxt-bool<,' it is founded on 

 De Bary's ' Vergleichende Anat. der Vegetations-organe,'' Cap. VI. 



