118 NEW ZEALAND PLANTS. 
In many streams the indigenous vegetation has been ousted by 
foreign competitors, and some beautiful flowering-plants have become 
established —e.g., the Cape pond-weed (Aponogeton distichum), an 
Australian water-lily (Ottelia ovalifolia), and the European water- 
buttercup (Ranunculus aquatilis). On the other hand, certain very 
troublesome weeds have become rampant, especially the watercress 
(Radicula Nasturtium aquaticum) and the American waterweed (Llodea 
canadensis). The watercress differs greatly in form both of leaf and 
flower from the aquatic form when growing on dry ground. 
Rivers, ponds, lakes, stagnant pools, moist soil, wet cliffs, and 
many other stations are the homes of the fresh-water algae, or the 
pond-scums, as one section of this important class of plants may be 
popularly called. These pond-scums frequently form green, slimy 
masses which float on the surface of the water. Common forms 
consist of what look like very fine, long, green hairs. Under a fairly 
strong power of the microscope these are seen to consist of long 
tubes, divided by thin walls into compartments, which contain plant- 
green, sometimes in the form of bands. 
The fresh-water algae are a very large group of plants, living under 
various conditions and of many forms, and, although occupying a low 
position in the plant-world, their structure is at times fairly com- 
plicated, and their methods of reproduction are quite elaborate. To 
this family belong the diatoms; the stoneworts (Chara, Nitella)— 
brittle plants up to a foot or more in height, with a peculiar odour, 
and growing submerged in extensive colonies on the beds of lakes and 
rivers; the desmids—tiny plants, often of star- or crescent-shaped 
solitary cells; the pond-scums, already mentioned; the red snow- 
plant; and many other groups. In the hot springs of the North 
Island are some peculiar forms, belonging to the blue-green algae, 
which are able to exist in water of a very high temperature. These 
were studied some years ago by Professor Setchell, of the University 
of California, and in a letter to the author he states that none of the 
New Zealand forms can endure a temperature greater than 167° 
Fahrenheit, which seems a bath quite hot enough in all truth! 
These hot-water algae are sometimes cited to show how living 
organisms could exist in the early days of the earth, when cold 
water would be unknown, and how such organisms may have per- 
sisted since those distant ages, and that they or their congeners may 
be the ancestors of the plants of the present day. 
