THE WATER-MILFOILS. Vue 
and P. pectinatus. The last-named sometimes occurs in_ brackish 
water, and its leaves are 2 to 4 inches long, extremely narrow or 
almost thread-like. 
The water-milfoils (Myriophyllum) differ from the pond-weeds mm 
that they have no floating leaves, but boldly raise the upper portion 
of their stems above the water-surface. They agree, however, in the 
fact that the aerial leaves, like the floating leaves of the pond-weeds, 
differ from the submerged ones. This is the more interesting because 
no line of demarcation on the erect stem separates the two kinds 
of leaves except the water-surface; that is to say, the same tissue 
can produce quite different leaf-forms according to change of outer 
cireumstances—a happening like that detailed for certain trees in 
Chapter V. The water-milfoils are graceful, feathery-looking plants, 
with four or more leaves frequently given off in whorls round the 
stem. The submerged leaves are cut into fine segments, a very 
common occurrence in many water-plants, whereas the aerial leaves 
are broader and much less cut. Some of the water-milfoils are 
bog-plants rather than aquatics—e.g., the small water-milfoil (J. 
Votschi). Another species of water-milfoil, the stout water-milfoil 
(M. robustum), attains 2 feet in height; but it is a swamp-plant, not 
a water-plant. Its stems are stout, erect, and branched, fitting it 
for a kind of life different from that of its aquatic more slender- 
stemmed relative the common water-milfoil (/. elatinoides) and its 
near relative M. propinquum. 
Certain plants are common in shallow mountain-streams. Such 
are: the water-chickweed (Montia fontana), which forms dense 
green masses or cushions of stems and leaves, and produces minute 
white flowers ; a large-flowered, quite showy willow-herb (Hpilobium 
macropus), which is dominant in many streams; and Schizeilema 
nitens, which in mountain-streams of certain parts of the South 
Island is greatly in evidence with its green shining mats of slender 
stems and minute leaves of three leaflets. 
It is probable that all aquatic seed-plants are descended from 
land-plants which took to the water by slow degrees, thanks to their 
innate plasticity, as their habitat gradually became wetter and wetter 
—a common enough circumstance. This is no place to discuss this 
question at length, but it may be pointed out that some plants can 
live equally well on land and in water, and even do not mind being 
submerged. The water-starworts (Callitriche) are examples. 
