124 Tue Ottawa NATURALIST. | September 
only) are to a great extent fed by natural springs from the moun- 
tains around and from the bottom of the lakes themselves ; conse- 
quently, when the ice forms, it is of very uneven thickness, and is 
therefore dangerous to travel on. I have noticed that the ice - 
formed at certain parts of the Pembina Lake would all melt away 
within 48 hours, although the weather kept much below freezing 
point. 
The ice forms on Pembina Lake, usually, between the 15th 
and 2oth November. It is a very few days earlier in small sur- 
rounding lakes. I have no personal accurate data as to when the 
ice breaks up in the spring. These lakes being land-locked and 
surrounded by high mountains, the ice becomes gradually honey- 
combed by the sun, and I have been told that even in the first 
days of May the waters are not entirely clear of ice. 
April, 1905. 
WHY OUR FIELD AND ROADSIDE WEEDS ARE IN- 
TRODUCED SPECIES. 
W. T. Macoun. 
A few years ago an Englishman was visiting Canada and, 
being observant, though not a botanist, was struck, when going 
about the cities and towns and along the country roads, by the 
similarity between the wild flowers he saw in Canada and those 
in the Old Country.* ‘‘ How is it,” he said to a well known 
Canadian botanist, ‘‘that, separated as we are by nearly 3,000 
miles of water, Canadian flowers are just the same as those at 
” 
home?” ‘ You do not see Canadian flowers,” said the botanist; 
‘* you see your own spe.ies which have been introduced into this® 
country and become weeds, as many of them are with you. 
* For instance, the following twenty-four well known common weeds are 
all introduced species: —Buttercup, Charlock, Shepherd’s-purse, Penny- 
cress, St. John’s-wort, Cockle, Chickweed, Purslane, Mallow, Purple- 
tufted Vetch, Mayweed, Yarrow, Ox-eye Daisy, Groundsel, Canada Thistle 
(which in spite of its name is a European plant), Chicory, Dandelion, Sow- 
thistle, Viper’s-bugloss, Bindweed, Mullein, Toad-flax, Heal-all and Plan- 
tain. 
