652 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Perfume of the Orchidese.* — M. E. Mesnard finds that the substance 
which gives the perfume to the flowers of Orchideae is generally localized 
in the epidermal cells of the inner surface of the petals or of the sepals, 
sometimes also in the cells of the outer surface. 
(3) Structure of Tissues. 
Secondary Tissues of Monocotyledons.! — t)r. D. H. Scott and Mr. 
G. Brebner have carried on a series of investigations, chiefly on the 
following points : — The development of the secondary tracheids in 
Yucca and Dracaena ; the secondary growth in thickness of the roots of 
Dracaena ; and the secondary growth in thickness of Aristea corymbosa 
(Irideas). 
Under the first head the point which the authors set themselves to 
determine was the nature of the water-conducting elements in the 
secondary wood ; whether, according to the older view, they are 
tracheids, i.e. single cells grown to an enormous length, or whether, 
according to the statement of some recent writers, they are true vessels 
resulting from cell-fusion. The observations of the authors, made on both 
Yucca and Dracaena , and both by microtome-sections and by maceration, 
completely established the older view. The elements in question are 
tracheids, formed by sliding growth, each arising from a single cell, which 
may grow to from 30 to 40 times its original length, but remains uni- 
nucleate throughout its whole development. 
In the adventitious roots of Dracaena the secondary growth in thick- 
ness starts from a number of distinct points, the chief formation of 
secondary tissue beginning at the base of rootlets. At the base of the 
rootlet the thickening takes place entirely by means of a pericyclic 
cambium. The connection between the vascular tissues inside and out- 
side the endoderm is maintained by special bundles which traverse the 
endoderm at various points. 
Aristea corymbosa forms an indefinite amount of secondary tissue by 
means of cambium which continues active during the whole life of the 
plant. The xylem of the secondary bundles consists chiefly of tra- 
cheids arising from the enormous elongation of a single cell. The 
cambium arises in the pericycle, and is a new formation. 
Curvature of the Cell-wall of the Endoderm of Roots.! — Herr A. 
Rimpach discusses the cause of the frequent curvature of the radial walls 
of the endoderm of roots, which he thinks cannot in all cases be assigned 
to the causes to which it has hitherto been attributed, viz. tensions which 
take place only in the preparation of the object, and decrease of volume 
accompanying suberization. His own observations and measurements 
lead the author to the conclusion that the phenomenon is due to a 
shrinking of the cell- walls caused by contraction of the root. This 
contraction is most marked in the basal portion of the root, diminishing 
towards the apex, and is much feebler in the secondary roots. It is 
frequently also altogether wanting in the primary root, especially in 
certain orders of Monocotyledons. 
* Comptes Rendus, cxvi. (1893) pp. 526-9. Cf. this Journal, ante, p. 214. 
t Ann. Bot., vii. (1893) pp. 21-62 (3 pis.). 
% Ber. Deutsch. Bot. Gesell., xi. (1893) pp. 94-113. 
