170 SUMMARY OF CURRENT RESEARCHES RELATING TO 



(3) Structure of Tissues. 



Differentiation of the Vascular Tissues in the Leaf and the 

 Stem.* — Pursuing his investigations on the continuity of the tissues in 

 the axile and appendicular organs of vascular plants, Prof. G. Bonnier 

 now brings forward evidenc- that the uniformity previously established 

 in the formation of the elements of the central cylinder in the root and 

 the stem occurs also in the leaves. The continuity is complete between 

 the tissues of the leaves and the corresponding tissues of the stem ; and 

 it is possible to regard the stem as formed from the concrescence of the 

 prolongations of the bases of the leaves. The leaves remain coherent to 

 one another by their bases, the union of which constitutes the stem ; the 

 vascular bundles combine ; and then, in the stem, the xylem is found 

 towards the centre, the phloem towards the periphery. Finally, when the 

 stem is transformed laterally or at its apex into organs destined for ab- 

 sorption- — the leaves — the poles of the xylem turn inwards in order to 

 place themselves in contact with the tissue which serves to draw up the 

 water from the soil, and we get the structure of the root. 



Common Origin of the Tissues of the Leaf and Stem in Phanero- 

 gams, f — L. Flot entirely supports the conclusions of Bonnier as to 

 the continuity of the tissues between the leaf and the bud on one hand, 

 aud between the bud and the stem on the other hand, and extends these 

 observations to the meristem of the vascular bundles and even to the 

 pith. It may be said that the apex of the stem, above where the first 

 leaves begin to show themselves, is composed simply by the juxtaposition 

 of segments each of which potentially encloses a leaf and its axillary 

 bud. The author disputes Baranetzky's assertion that the arrangement 

 of the initial layers has no necessary influence on the nature or ultimate 

 differentiation of the tissues. 



Conducting Tissue in the Ovary of Orchidese. % — W. Busse has 

 studied the nature of the tissue which supplies the pollen-tubes with 

 food-material in the form of soluble carbohydrates in the Orchidere, 

 especially in species of Vanilla. It is formed either by a modification 

 of the inner epiderm of the ovary alone, or of two or three layers of 

 hypodermal cells as well ; and consists usually of six distinct streaks, 

 the cells of which are distinguished by their small size, thick walls, and 

 abundant protoplasm. It is formed before the opening of the bud, com- 

 mencing with the apex of the ovary, and advancing gradually towards 

 its base. The cuticle becomes detached from the epiderm, and more or 

 less disorganised before the arrival of the pollen-tubes in the cavity of 

 the ovary. The outermost layer of cells is used up first of all for the 

 nutrition of the pollen-tubes, and then the lower ones. The production 

 of the soluble carbohydrates is effected by an enzyme. The conducting 

 tissue is localised in those spots in the wall of the ovary where it will 

 best facilitate the act of impregnation. 



Succulent Plants. § — W. Brenner has studied the anatomical struc- 

 ture of several species of Sedum, Sempervivum, and Mesembryanthemum. 



* Comptts Kendus, cxxxi. (1900) pp. 1276-86 (8 figs.). Cf. this Journal, ante, 

 p. 51. t Tom. oit, pp. 1319-22 (3 figs.). 



X Bot. Centralbl., Ixxxiv. (1900) pp. 209-22. 

 Flora, lxxxvii. (1900) pp. 387-439 (15 figs.). 



