1897] CURRENT LITERATURE 299 
basswood, etc.) migrate more slowly, and are yet far from their rightful 
limits. Some of the trees of the second class have been transplanted success- 
fully far to the north of their natural habitat. 
The conditions governing distribution are listed as follows: (1) Distance 
or proximity of the sea. Most tree lines are more or less parallel to the 
Atlantic ocean and Hudson bay, as a rule appearing to shun the salt water. 
Pinus Banksiana, for instance, never comes to the coast, but appears almost 
everywhere else, even having outliers in the center of Nova Scotia and New 
Brunswick. Populus balsamifera, however, seems to love the sea, and is 
absent from a vast area in the interior. (2) Geological changes in arrange- 
ment of land and water. Some erratic lines, seen especially in the balsam 
poplar, are probably due to peculiarities in the distribution of glacier ice. 
Thuya occidentalis has the most remarkable line of all, turning abruptly 
southward both in the east and west; the cause in the east is thought to be 
the recent insular condition of Nova Scotia. This species has a large outlier 
300 miles north of its regular line. (3) General dryness or moisture. Dr. 
Bell makes a strong argument for drought as the cause of treeless prairies 
rather than forest fires; the tree lines are not sharp but concentric, and the 
more hardy trees have their lines farther out into the dry region. (4) 
Extremes of heat and cold. (5) Local heat and moisture from lakes and 
rivers. This usually results in a further extension northward along river 
Courses than on the uplands. (6) General elevation above the sea. This 
may account for the absence of the elm and black ash in a large interior area, 
since they occur on all sides of this region. (7) Local elevations. (8) Local 
depressions. (9) Diseases and insect pests. (10) Rapid or slow means of 
dispersion. (11) Forest fires. These are said to be frequent as natural 
phenomena. After a fire there springs up a low shrub and herb vegetation, 
then poplars, birches, and willows in about 20 years. Conifers dominate 
again in 50 years and reach maturity in about 150 years.—H. C. C. 
THE WEEDs of Canada are treated briefly in a bulletin (no. 28) of 39 
pages from the Central Experimental farm, prepared by Dr. James Fletcher. 
General methods for the control and extermination of weeds are given, fol- 
lowed by descriptions and illustrations of the following fourteen species of 
‘omanid 9 ossessing special interest: Sisymbrium altissimum L., Arabis hirsuta 
= Conringia orientalis Andrz., Neslia paniculata Desr., Thlaspi arvense 
ne Lepidium apetalum Willd., Safonaria Vaccaria L., Silene Cucubalus 
Wib., Hieracium aurantiacum L., Cynoglossum officinale L., Echium vulgare 
p” Sakola Kati Tragus Moq., Rumex crispus L., and Hierochloa borealis 
R.&S. The first six species belong to the Crucifere. Much information 
oe the more prominent Canadian weeds, numbering over one hundred 
caer Species, is thrown into tabular form and made easy of reference.— 
