Flora Unveiled

Flora’s Secret Gardens j 483

483 483

possess two types of gametophytes: a male gametophyte containing antheridia, and a female

gametophyte containing archegonia. Cryptogams with only one type of sporangium and

one type of spore that produces one type of gametophyte containing both antheridia and

archegonia (monoecious gametophytes), are termed homosporous. Cryptogams producing

two types of sporangia, two types of spores, and separate male and female gametophytes

(dioecious gametophytes), are termed heterosporous.

Heterospory in the spikemoss Selaginella,^12 a fern ally, is illustrated in Figure 18.3. The

Selaginella sporophyte (left) produces leafy branches, some of which terminate with cone-

like reproductive structures called strobili. These strobili consist of small, spore- bearing

leaves called sporophylls, each one bearing either a female sporangium (megasporangium) or

a male sporangium (microsporangium) (see Figure 18.3).^13

The megasporangium of Selaginella produces female megaspores, and the microsporan-

gium produces male microspores. These two types of spores then undergo cell divisions to

form the mature female and male gametophytes, respectively. The megaporangium contains

several archegonia, each with an egg at its base, whereas the microsporangium forms a single

antheridium that gives rise to flagellated motile sperm cells.

The male and female gametophytes of Selaginella usually fall to the ground near one

another. In damp or rainy weather, the motile sperm cells swim over to the female gameto-

phytes, swim down the neck of the archegonia, and fertilize the egg cells, thus forming the

zygote. The zygote then grows into another sporophytic Selaginella plant.

Hofmeister realized that heterospory was the crucial morphological link uniting the life

cycles of the cryptogams and the seed plants. In flowering plants, the egg- containing embryo

sacs and the sperm- containing pollen grains are produced by two separate structures, the

ovules and anthers. Hofmeister reasoned that if the two sexual structures of angiosperms

Figure 18.3 Reproduction in the spikemoss, Selaginella. (Left) Leafy branches and cone-

like strobili of Selaginella sporophyte. (Middle) Strobili showing microsporangia containing

microspores and megasporangia containing megaspores. (Right) Sperm released by the antheridia

of the microgametophyte fertilize the egg in the archegonium of the megagametophyte, giving rise

to a new sporophyte.