The species of seaweeds that are known and classified are said to number several thousands. These plants, which have neither vessels for the conduction of fluids, nor fibers, consisting simply of the first vegetable element, the cell, have, notwithstanding this limitation, assumed a great variety of forms. In size they vary from one one-thousandth of an inch in diameter, the smallest green plants known, to those which exceed in length the height of the tallest trees and form dense submarine forests, which in places make comparatively deep water impassable for boats. In texture they vary from a jelly- to a paper- and a leather-like consistency. In color they have all the shades of green, brown, and red. DIATOMS AND OTHER MINUTE ALGÆ Among the smallest algæ are diatoms. They are microscopic in size, but exist everywhere in both salt and fresh water, and are infinite in variety as well as in numbers. They have a silicious, shell-like covering, which divides and subdivides in their reproductive growth, forming varied shapes which are exceedingly beautiful and interesting to examine under the microscope. In vast numbers they float on the surface of the sea, and, together with other minute free-floating organisms, form the basis of food-supply for fishes. Their indestructible shells fall to the bottom of the sea, forming large deposits, which in time become fossilized. The city of Richmond, Virginia, is built upon [pg033] a fossiliferous bed of diatoms, which measures twenty to eighty feet in depth and several miles in length. Associated with diatoms, in fresh water, are desmids, which are green in color and resemble the diatoms except in having a cartilaginous instead of a silicious covering. Another minute organism, /Pyrocystis noctiluca/, is luminous and is said to produce the beautiful phosphorescent effects seen in tropical seas. /Trichodesmium/ is a little alga which periodically occurs in great numbers, giving the water a red appearance, as in the Red Sea, which is said to derive its name from this circumstance. RED SNOW In the high latitudes of the arctic regions, also on snowy mountains at altitudes where all vegetable life is supposed to be extinguished, there sometimes appears a redness on the surface of the snow, which in some cases extends for many miles. At a certain place in Greenland the color was so vivid that an arctic voyager named the locality the Crimson Bluffs.