Page:Popular Science Monthly Volume 15.djvu/760

 —a fact of great importance in its bearing on the phenomena of inheritance. Recent researches, indeed, render it almost certain that fertilization, whether in the animal or the vegetable kingdom, consists essentially in the coalescence and consequent loss of individuality of the protoplasmic contents of two cells.

In by far the greater number of plants the protoplasm of most of the cells which are exposed to the sunlight undergoes a curious and important differentiation, part of it becoming separated from the remainder in the form usually of green granules, known as chlorophyll. The chlorophyl-granules thus consist of true protoplasm, their color being due to the presence of a green coloring matter, which may be extracted, leaving behind the colorless protoplasmic base.

The coloring matter of chlorophyl presents under the spectroscope a very characteristic spectrum. For our knowledge of its optical properties, on which time will not now permit me to dwell, we are mainly indebted to the researches of your townsman, Dr. Sorby, who has made these the subject of a series of elaborate investigations, which have contributed largely to the advancement of an important department of physical science.

That the chlorophyl is a living substance, like the uncolored protoplasm of the cell, is sufficiently obvious. When once formed, the chlorophyl-granule may grow by intussusception of nutriment to many times its original size, and may multiply itself by division.

To the presence of chlorophyl is due one of the most striking aspects of external nature—the green color of the vegetation which clothes the surface of the earth: and with its formation is introduced a function of fundamental importance in the economy of plants, for it is on the cells which contain this substance that devolves the faculty of decomposing carbonic acid. On this depends the assimilation of plants, a process which becomes manifest externally by the exhalation of oxygen. Now, it is under the influence of light on the chlorophyl containing cell that this evolution of oxygen is brought about. The recent observations of Draper and of Pfeffer have shown that in this action the solar spectrum is not equally effective in all its parts; that the yellow and least refrangible rays are those which act with most intensity; that the violent and other highly refrangible rays of the visible spectrum take but a very subordinate part in assimilation; and that the invisible rays which lie beyond the violet are totally inoperative.

In almost every grain of chlorophyl one or more starch-granules may be seen. This starch is chemically isomeric with the cellulose cell-wall, with woody fiber, and other hard parts of plants, and is one of the most important products of assimilation. When plants whose chlorophyl contains starch are left for a sufficient time in darkness, the starch is absorbed and completely disappears; but when they