SCIENTIFIC SUMMARY. 
429 
growth. . . . These winds seem to act in two ways. First, by their 
drying power, as seen in the absence of trees on slopes of hills exposed to 
them, while trees may abound on the opposite slope, though facing the 
south and more exposed to the sun. Second, by their coolness not per- 
mitting the sun’s heat to produce the growth, even where moisture is 
abundant.” The grouping of the trees of the district at different distances 
around San Francisco is then given, in more detail. 
The Structure of Bark beneath the Microscope. — Dr. Braithwaite, F.L.S., 
who has a paper of some considerable length in the last u Journal of the 
Quekett Club,” on the vegetable tissues generally, makes the following 
observations on the subject of bark. He says that after the production of 
more or less numerous cork lamellae, the phellogen dies or loses its vital 
activity, but a development of secondary cork tissue^akes place within the 
bast part of the vascular bundle, in the form of tangential rows of tabular 
cork cells, which loosen from the growing outer part of the vascular bundle.. 
The cork lamellae, as it were, cut out and force off from the rind flat pieces in 
form of scales or rings ; all this outer part is dead, and the process, oft 
repeated from the circumference of the stem, causes the new cork lamellae 
to become gradually imbedded more deeply in the growing cortical tissue,, 
and we get a constantly thickening peripheral layer of dry tissue separating 
from the living part of the rind; this is the bark. The condition 
is very evident in the large scales of bark in Plantanx orientals or 
sycamore, and in old stems of the Pinus sylvestris or Scotch pine, and in the 
ringlike bands of the cherry-tree. In the oak, lime, poplar, elder and 
horse-chestnut, similar plates of thin-walled cells arise in the interior of the 
bast bundles, but the old dried scales do not fall off, but tear only at the 
margins in a longitudinal direction, so that the stem becomes clothed with 
bark consisting of several dead scales lying under each other, presenting: 
internally all the elements of bast, and externally primary cork tissue. In. 
the pine and larch we have a fissured periderm, like that of the horse-chest- 
nut, and in the pine consisting partly of thin-walled and thickened cells in- 
alternate layers, but the conifers are specially remarkable for the presence 
of a spurions large-celled parenchym tissue, which appears between the 
periderm layers, and separates the elements of the bast bundle into smaller 
or larger groups. 
Physiologicul Groups in the Vegetable Kingdom. — In the “ Archives des 
Sciences ” for May 1874, M. A. De Candolle makes an attempt to classify 
plants according to the climate they inhabit. He sketches five groups. 
First, plants requiring a great amount of heat and moisture. A name ex- 
pressive of both these requirements would be cumbrous ; so he chooses for 
this group a name referring to the temperature only, and calls them fffe- 
gathermal plants, or in short Megatherms, i.e., plants to which much heat, 
is essential. These inhabit the rainy intertropical regions in the plains and 
sultry valleys up to the 30th parallel. Second, plants requiring about as 
much heat but far less moisture. These, taking the name from the latter 
characteristic, he terms Xerophilous plants, lovers of dryness. They are 
pretty widely distributed, but they especially affect the regions bordering 
the tropics, and extending, say to the 35th parallel in both hemispheres.. 
The third group, Mesothermal plants, require, as the name denotes, moderate- 
