Elementary botany . tary spo-rangia become changed into the protonema. In some otherferns, as in asplenium(A. filix-fcemina, var. clarissima), prothalliaare borne among the aborted sporangia, which bear antheridiaand archegonia. In these cases the gametophyte develops fromthe tissue of the sporophyte without the intervention or necessityof the spores. This is apospory. 685. The sporophyte may develop directly from the tissueof the gametophyte.—In some of the ferns, Pteris cretica forexample, the embryo fern sporophyte arises directly from the tissueof the prothallium, withoutthe intervention o
Elementary botany . tary spo-rangia become changed into the protonema. In some otherferns, as in asplenium(A. filix-fcemina, var. clarissima), prothalliaare borne among the aborted sporangia, which bear antheridiaand archegonia. In these cases the gametophyte develops fromthe tissue of the sporophyte without the intervention or necessityof the spores. This is apospory. 685. The sporophyte may develop directly from the tissueof the gametophyte.—In some of the ferns, Pteris cretica forexample, the embryo fern sporophyte arises directly from the tissueof the prothallium, withoutthe intervention of sexualorgans, and in some casesno sexual organs are de-veloped on such organs, then, andthe fusion of the spermato-zoid and egg nucleus arenot here necessary for thedevelopment of the spo-rophyte. This is occurs in someother species of ferns, andin other groups of plants as well, though it is in general a rareoccurrence except in certain species, where it may be the Fig. in Pteris cretica. GAMETOPHYTE AND SPOROPHYTE. 347 686. Types of nuclear division.—The nuclear figures in thevegetative cells are usually different from those in the sporemother cells. In the spore mother cells there are two types ofnuclear division, (i) The first division in the mother cell iscalled heterotypic. The early stages of this division usuallyextend over a longer period than the second, and the figures aremore complex. Before the chromosomes arrive at the nuclearplate they are often in the form of rings, or tetrads, or in theform of X, V, or Y, and the number is usually one half the num-ber in the preceding cells of the sporophyte. (2) The homo-typic division immediately follows the heterotypic and the figuresare simpler, often the chromosomes being of a hook form, orsometimes much stouter than in the heterotypic division. Inthe vegetative cells (sometimes called somatic cells, or bodycells in contrast with reproductive cells) there is
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