=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. [Illustration: Fig. 301. Spores of asplenium; exospore removed from the one at the right.] [Illustration: Fig. 302. Germinating spores of Pteris aquilina still in the sporangium.] [Illustration: 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. =554. Prothallium.=—Very soon, if the sowing has not been so crowded as to prevent the young plants from obtaining nutriment sufficient, we will see that the end of this protonema is broadening, as shown in fig. 303. This is done by the formation of the cell walls in different directions. It now continues to grow in this way, the end becoming broader and broader, and new rhizoids are formed from the under surface of the cells. The growing point remains at the middle of the advancing margin, and the cells which are cut off from either side, as they become old, widen out. In this way the “wings,” or margins of the little, green, flattened body, are in advance of the growing point, and the object is more or less heart-shaped, as shown in fig. 297. Thus we see how the prothallium of ferns is formed. =555. Sexual organs of ferns.=—If we take one of the prothallia of ferns which have grown from the sowings of fern spores, or one of those which may be often found growing on the soil of pots in conservatories, mount it in water on a slip, with the under side uppermost, we can then examine it for the sexual organs, for these are borne in most cases on the under side. [Illustration: Fig. 304. Male prothallium of a fern (niphobolus), in form of an alga or protonema. Spermatozoids escaping from antheridia.]