82 SUMMARY OF CURRENT RESEARCHES RELATING TO 
aluminate ; these arise from the decomposition in the soil of argillaceous 
substances in the presence of organic acids, the result being the produc- 
tion of silica and of silicates of the alkalies and alkaline earths, which 
are also in part absorbed by the plant. 
(3) Structure of Tissues. 
Regulatory Formation of Mechanical Tissue.* — Prof. F. C. New- 
combe points out that the mechanical theory of growth rests partly on 
two erroneous assumptions, viz. : — that extension of cells is due wholly 
to stretching from the hydrostatic pressure of the cell-contents; and 
that tbe resistance offered by the cortex is great enough to control the 
amount and direction of the growth from the cambium. All the phe- 
nomena of growth can be better explained by the theory of self-regula- 
tion. As examples of regulatory growth may be cited the excessive 
formation of mechanical tissue in stems swayed by the wind ; the growth 
of clasping organs when they support a weight ; and the great multipli- 
cation of mechanical tissue in roots, stems, and petioles, when subjected 
to an artificial traction. 
Hygroscopic Tissue of the Pappus of Composite, f — Herr W. 
Taliew classifies the various forms of this tissue under three principal 
types, viz. : — (1) The Lactuca type (all Cichoriaceae examined, and most 
Tubuliflorse). In the upper part of the fruit the pericarp forms a 
hollow beak, the hollow formed by the disappearance of parenchymatous 
tissue being closed above by a corky diaphragm. At the upper end of 
the beak is the hygroscopic tissue in the form of a continuous ring. 
Water causes the walls of this tissue to swell greatly in the superficial 
direction. The tissue which bears the pappus is fixed to the upper 
margins cf the hygroscopic ring, and its hairs are consequently set in 
motion by the moistening or drying of this ring. This may take 
place for a very considerable time (even ten years) after the ripening of 
the fruit. (2) The Tussilago- type (Tussilagineee). The movement of 
the pappus is brought about by the unequal capacity of swelling of the 
inner and outer sides of the tissue on which the pappus is seated. 
(3) Cirsium-type (all Cynareae examined). The movement takes place 
in the hairs of the pappus themselves, which curve actively, especially 
in their lower part. Transitional forms occur between the various 
types. 
In Lactuca Scariola the hygroscopic tissue is fully developed on the 
first day after the opening of the flower. The involucral scales exhibit, 
in many cases, hygroscopic properties corresponding to those of the 
pappus, but less intense. The greatest degree of sensitiveness occurs 
in the Tussilagineae. 
The author states that the variation in the hygroscopic mechanism 
does not correspond with the usual classification of the Composite into 
tribes, but more closely with that of Hoffmann in Engler and Prantl’s 
4 Natiirliche Pflanzenfamilien.’ 
Spring and Autumn Wood. — Herr K. G. Lutz { attributes the 
difference between the spring and autumn wood (beech and pine) to the 
* Bob Gazette, xx. (1895) pp. 441-8. 
t In Russian; 39 pp. and 1 pi ., Kazan, 1894. See Bot. Centralbl., lxiii. (1895) 
p. 320. + Ber. Deutseh. Bot. Gesell., xiii. (1895) pp. 185-8. 
