ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
85 
under the same conditions, the plant remains altogether green. In 
some the inflorescence is hardly affected by shade ; in others the number 
of flowers is diminished, and in certain Composite the number of capitula. 
The effect of shade is always seen in a diminution of the reproductive 
power, either in a smaller number of seeds in the fruit, or in abortion of 
the fruits themselves. 
Influence of External Conditions on Growth.* — Pursuing his inves- 
tigations on the influence of the moisture of the soil on the growth of 
plants, M. E. Gain states that the power of absorption of water depends 
on the anatomical structure of the axial organs ; and this is influenced 
by the nature of the soil. In a dry soil the fibro-vascular bundles are 
more numerous and further apart, ensuring a more perfect irrigation of 
the tissues ; while in a moist soil the conducting tissue is protected ex- 
ternally by a sclerenchymatous layer. In a dry soil there is often a 
thick and continuous hypodermal collenchyme. 
Influence of the Pressure of Gases on Growth-t — From a series of 
experiments made on about 50 different species, M. P. Jaccard states 
that a moderate increase beyond the normal pressure of the air, to the 
extent of from 3-6 atmospheres, promotes the growth of plants contain- 
ing chlorophyll ; and this is still more strikingly the case with a mode- 
rate decrease of pressure. An excessive change, on the other hand, in 
either direction retards growth. The results are somewhat different 
when the air is super-oxygenated. Very little morphological or his- 
logical change is caused by a moderate increase of pressure. 
Unequal Resistance of Plants to Drought.^ — M. P. P. Deherain 
shows that different plants — wheat and rye-grass — have a very different 
power of resistance to long-continued drought, from the difference in 
their capacity of putting out long roots which strike into the lower and 
moister layers of soil. The former of these two plants has a great 
advantage over the latter in this respect. 
Cell-union in Herbaceous Grafting.§ — Mr. J. S. Wright has pro- 
duced successful grafts between the following herbaceous plants : — 
tomato on tomato, potato on tomato, geranium on geranium, geranium 
on tomato, (tomato on geranium failed), cactus on cactus, Tradescantia 
zebrina on itself and on tomato, the last an instance of grafting a 
monocotyledonous scion on a dicotyledonous stock. He states that in 
herbaceous grafting the union occurs in one of two ways ; either by long 
exerted pressure holding old cell-walls together, and gradually causing 
them to cohere permanently ; or through the development of meriste- 
matic tissue by one or both members of the graft, after which the boundary 
walls meet and unite through pressure. 
Growth of the Hoot. || — Prof. A. Letellier propounds the following 
rules as governing the growth and direction of the root. The direction 
taken by the young portion of the root is that which corresponds to its 
* Bull. Soc. Bot. France, xl. (1893) pp. 142-5. Cf. this Journal, 1893, p. 356. 
t Rev. Gen. de Bot. (Bonnier), v. (1893) pp. 289-302, 348-54, 382-8 (4 pis.) ; and 
Comptes Rend us, cxvi. (1893) pp. 830-3. 
X Comptes Rendus, cxvii. (1893) pp. 269-72. 
§ Bot. Gazette, xviii. (1893) pp. 285-93 (2 pis.). 
|| Essai de statique vegetale. See Bull. Soc. Belg. Micr., xix. (1893) p. 131. 
