204 D. H. SOOTT. 



To these points of difference may probabl}' be added the 

 absence of anastomoses in the case of the cells. 



Taking all these facts into consideration it appears neces- 

 sary, at least until transitional forms have been shown to occur, 

 to regard the two categories of laticiferous tubes as distinct 

 forms of tissue, which are only physiologically related to one 

 another. And if this be so, it will follow, as suggested above, 

 that the formation of laticiferous tissue is a phenomenon 

 which has appeared independently, at least twice, within the 

 circle of relationship now represented by the natural order 

 Euphorbiacese.^ 



EXPLANATION OF PLATE XVII. 



Illustrating Mr. D. H. Scott's Memoir on the "Laticiferous 

 Tissue of Man i hot Glaziovii (the Ceara Rubber.)" 



Fig. 1. — Hypodermal laticiferous tissue from young stem. Tangential 

 section, x 3G0. /. Laticiferous vessels. Many of the parenchymatous cells 

 contain clustered crystals of calcium oxalate, indicated at c. 



Fig. 2. — Laticiferous tissue from the primary phloem of a stem about 1 cm. 

 thick. Tangential section, x 270. /. Laticiferous vessels, t. Their trans- 

 verse connecting branches, p. Parenchyma. 



Fig. 3. — Portions of young laticiferous vessels from an inner layer of the 

 secondary phloem. Tangential section, x 4G0. At t and t' the transverse 

 walls are preserved. At i" a rim of cellulose marks the position of an absorbed 

 transverse wall. Several perforations in the lateral wall are visible. lu the 

 vessel to the right the transverse walls have almost disappeared, m. liledul- 

 lary ray. 



Fig. 4. — Tangential section through the primary phloem of a young stem, 

 showing an " anastomosing knot " between the laticiferous vessels, x 360. 

 The continuation of the vessel ending at a lay in a different plane from that of 

 ihe section, m parenchyma resembling medullary rays. Its cells contain 

 starch, which is indicated in two of (he cells. 



As an example of a phenomenon which has probably appeared repeatedly 

 within a group of moderate extent, that of Heterospory may be cited. Cf. 

 Goebel, ' Grundziige der Systeniatik,' p. 313, &c. 



