FRESH-WATER ALGAE. 161
but not infrequently they deviate sideways without apparent cause, and after
deviating, return regain. They double round projecting objects or push them
out of the way in a surprising manner, and in short, behave exactly like
animals. And yet no one has been able to discover how this movement is
produced. Endless theories have been propounded, but nothing has been
satisfactorily proved, and the movements of the Diatoms must still be
numbered among the unsolved problems which nature presents to the
student.
Desmids also possess the faculty of locomotion, but in a much more
feeble degree. Compared with Diatoms the most active Desmids are slow
and sluggish, and in the majority, the movement is scarcely if at all percepti-
ble. The motion is usually slow and creeping, and some forms (such as
Closterium and Penium) have a curious rotating movement, one end remaining
fixed while the other oscillates.
Time will not allow me even to mention a host of interesting and beautiful
microscopic plants of unicellular structure which abound in stagnant pools
and moorland ditches. Indeed the subject of the Freshwater Algae is so
extensive that many evenings could very profitably be devoted to them, taking
one or two sections at a time. All I can do to-night is to skip from one group
to another in an erratic and superficial way, taking examples almost at
random, without any definite scientific arrangement.
Before leaving the unicellular algae there is just one point to be noted.
What we commonly regard as a plant does not always consist of one detached
cell only ; in certain groups a number of single cells are united in a common
investment or joined together by protoplasmic processes into a family or
colony, which is technically' termed a caenobium ; and this colony in most
cases assumes a symmetrical form. The number of individuals composing
such a colony varies greatly, Pediastrum consists of about a dozen polygonal
cells which form a flat plate-like disc, the cells of the outer row or rim being
drawn out into lobes or teeth. This is a small colony compared with Volvox,
which though only about one millimetre in diameter, is composed of from
10,000 to 20,000 individual cells. In the alga called the "Water-net" or
Hydrodictyon, the number of cells is indefinite—they are joined end to end
and form the sides of the honeycomb network.
A large tribe of Algae, composing the class Cyanophyceae are charac-
terized by the absence of chlorophyll, or pure green colouring matter, the
prevailing hue of the cell-contents being steel blue or blue green in various
shades of intensity. As you all probably know the classification of the
seaweeds into their principal divisions, mainly follows the colour of the
plants. Thus we have the Green Algae, the Brown Algae, the Red or Pink
Algae, and the Blue-green Algae. Colour, however, is in itself of no systematic
importance—many systematists ignore it almost entirely—but it so happens
that differences in colour generally (though not invariably) coincide with
important morphological distinctions.
In the Blue-green Algae we find the cells filled with pale bluish
protoplasm, never pure grass-green as in the Desmids. Some are terrestrial
and some aquatic, and generally when out of the water they become invested
in a gelatinous envelope, and may then resemble a mass of jelly. A familiar
example of this class is the bladdery india-rubber-like Nostoc which makes its