260
THE ESSEX FIELD CLUB.
is the cause of this difference of degree on the part of the style and stigma in
favouring the formation of pollen-tubes. This can only be arrived at after a
very extended series of experiments; it is important to ascertain to what extent
the comparatively few experiments instituted by Strasburger hold good, as to
how many different kinds of pollen will form tubes on foreign stigmas, also to
what extent reciprocity holds good. Cut flowers can be used, and the anthers
may be cut away so as not to interfere with the experiments, although this is not
essential, as Strasburger has shown that both normal and foreign pollen often
germinate side by side on the same stigma. It is advisable to use pollen for
experimenting that possesses marked microscopic characters, so as to be able to
trace its tubes readily when a section of the style is placed under the microscope.
In this connection, experiments between wind-fertilized and insect-fertilized plants
would suggest themselves as being eligible—the pollen, as a rule, being very
dissimilar in the two groups. Finally, Strasburger has shown that the stigma is
not absolutely necessary, the pollen germinating quite as readily on the cut
surface of the style after the stigma had been removed, as on the stigma itself.
Pollen-grains, as a rule, form their tubes within twenty-four hours after being
placed on the stigma. To ascertain whether pollen-tubes have been emitted, it is
necessary to make longitudinal sections of the stigma and style. With a dilute
watery solution of magenta, the pollen-tubes stain deeply, the tissue of the style
being only faintly tinged.
Views as to the Origin of Multicellular Plants,—In some of the lower forms of
life, the cells are naked—not furnished with a cell-wall—as in the fungi known as
the Myxogastres during the vegetative period. In others again, as Pleurococcus
vulgaris (the green matter on trunks, stones, and almost everything that has been
exposed for any length of time to the atmosphere), the protoplasm or vital part of
the single cell which constitutes the plant, is enclosed in a protective skin, or
cell-wall, of cellulose, secreted by the protoplasm. This cell-wall has usually been
considered as the outermost coating of the cell, but it has recently been
shown3 that there is present in many species of algae, both fresh-water and marine,
a thin layer external to the true cell-wall, agreeing in many respects, chemically and
physically, with protoplasm. This layer is generally more or less viscid, especially
when the cells are young, and it has been suggested that it has to do in some way
with the evolution of multicellular from unicellular plants. This change is
effected by degrees. It is known that many normally unicellular fresh-water algae
form "colonies"—the unicellular organisms retaining their individuality so far as
nutrition and reproduction are concerned, but adhere in colonies, being bound
together by a matrix of viscid matter outside their cell-walls. There are various
advantages connected with this colony-forming idea, one being that of greater
power of resisting desiccation—possibly the greatest of the many difficulties
encountered by the pioneers of plants in attempting to emerge from their
primitive aquatic mode of life, and take possession by degrees of dry land—and
it is perhaps not too much to say that the idea of colony-formation on the part of
lowly plants has resulted in the immense variety of vegetable life which to day
clothes the earth. In many of the simpler algae, which may be considered the
starting point of plant life, the usual mode of reproduction is by fission, or a gradual
contraction of the parent cell into two parts, each destined under favourable condi-
tions to repeat the same process, and after the external secretion phase had been
perfected, the unicellular or multicellular nature of the plant depended to a very great
8 Berthold; Unters. Bot. Inst. Tubingen, ii. (1886), Massee ; Journ. Bot., Sept., 1887.