Fig. 300.
Spore crushed to remove exospore
and show endospore.
551. Spores.—We can easily obtain material for the study of the spores of ferns. The spores vary in shape to some extent. Many of them are shaped like a three-sided pyramid. One of these is shown in [fig. 298]. The outer wall is roughened, and on one end are three elevated ridges which radiate from a given point. A spore of the Christmas fern is shown in [fig. 299]. The outer wall here is more or less winged. At [fig. 300] is a spore of the same species from which the outer wall has been crushed, showing that there is an inner wall also. If possible we should study the germination of the spores of some fern.
552. Germination of the spores.—After the spores have been sown for about one week to ten days we should mount a few in water for examination with the microscope in order to study the early stages. If germination has begun, we find that here and there are short slender green threads, in many cases attached to brownish bits, the old walls of the spores. Often one will sow the sporangia along with the spores, and in such cases there may be found a number of spores still within the old sporangium wall that are germinating, when they will appear as in [fig. 302].
Fig. 301.
Spores of asplenium;
exospore removed from
the one at the right.
Fig. 302.
Germinating spores
of Pteris aquilina
still in the sporangium.
Fig. 303.
Young prothallium of a fern (niphobolus).
553. Protonema.—These short green threads are called protonemal threads, or protonema, which means a first thread, and it here signifies that this short thread only precedes a larger growth of the same object. In figs. [302], [303] are shown several stages of germination of different spores. Soon after the short germ tube emerges from the crack in the spore wall, it divides by the formation of a cross wall, and as it increases in length other cross walls are formed. But very early in its growth we see that a slender outgrowth takes place from the cell nearest the old spore wall. This slender thread is colorless, and is not divided into cells. It is the first rhizoid, and serves both as an organ of attachment for the thread, and for taking up nutriment.