OBSERVATIONS FOR YOUNG BOTANISTS 71 
vertically and are prevented from being as fully formed as in air. 
Thus, when the leaves of the arrow-head reach the surface, 
they produce blades, at first oval, then like an arrow-head, 
rising out of the water. 
Fleshy Type. — Plants which grow in very dry and hot 
localities are obliged to store up water for several months, when 
little or no rain falls. Such plants are well seen in desert 
regions — as the cactuses of Mexico, the euphorbias of the 
Soudan, &c. — but we have several natives which do thus, as the 
sedums and house-leeks. These leaves have very thick skins, 
and so lose very little water by transpiration. 
Juniper-like Type. — Most gardens contain shrubs of the 
juniper, cypress, or tliuja kind, remarkable for their extremely 
minute leaves closely pressed against the twigs. This is a 
result of growing in dry mountainous regions ; for there are 
some kinds of Veronica (our English blue-flowered speedwell is of 
the same genus) which are evergreen shrubs in New Zealand ; 
and those which grow high up on the mountains have precisely 
similar foliage. 
These few examples will show the young student how he 
should observe that the numerous forms and characteristics of 
leaves depend entirely upon the external conditions in which 
they and their ancestry have grown. Thus trees, as oaks, which 
shed their leaves in England, retain them a much longer time in 
South Europe, and may become evergreens where there is no 
severe winter'climate like ours. 
He may also notice how monocotyledons have mostly long 
leaves with parallel veins, as is easily seen in a blade of grass. 
Now we have seen how a ribbon-like leaf is one type of sub- 
merged leaves. This fact, coupled with a large number of others, 
has rendered it extremely probable that all monocotyledons have 
descended from aquatic dicotyledons. Several, of course, are 
now terrestrial, but they still retain many features of an 
“ aquatic ” character ; e.g., notice that the skin of a banana 
shows a network of square meshes inside it. A similar network 
is seen in oval aquatic leaves of monocotyledons. 
Metamorphosis of Le.wes. — Leaves are often used for 
other purposes than their natural one, viz., that of assimilat- 
ing carbonic-acid gas from the air ; thus tendrils are often made 
out of them, as in the pea family, such as peas and vetches. 
In these tendrils the little sensitive branchlets can be seen to 
correspond with the “ mid-ribs” of the leaflets. 
In Clematis, however, or the traveller’s joy of our hedges, 
the leaflets are retained, and it climbs by means of the leaf-stalks 
or petioles. Experiments should be made in placing a stick so 
as to touch a tendril and examining it after a few hours, when it 
will be found coiling round it. 
Stipules. — These really represent a pair of leaflets ; but 
when, they are unattached to the leaf-stalk, from which they 
really arise, the union is lost to sight within the stem itself, so 
