ASEXUAL REP ROD UC TION. 



177 



reproduction of the protoplasm and its nuclear elements, or in short of 

 the cells; all reproduction (excluding the important fact of fertilization) 

 is growth. The ovum, asexually produced from the parent ovum or 

 its lineal descendant cells, grows and reproduces itself in turn, building 

 up the embryo. The embryo grows into an adult organism, and the 

 surplus of continued growing energy results in the asexual production 

 of buds, or the sexual discharge of differentiated reproductive elements. 

 We start from the ordinary processes of cell-multiplication and 

 regeneration exhibited in the normal organism. Then come the 

 processes by which lost members are regenerated, involving more or 

 less serious extra growth. To these we must add the rarer and yet 

 not rare cases where the artificial halves or fractions of an organism 

 can grow into wholes. Normal and frequent, however, are the very 

 abundant cases of budding, where a sponge or hydra, zoophyte or 

 coral, has surplus enough to grow off new individuals, which remain 

 continuous with itself. The parent organism, whether zoophyte or 

 strawberry plant, has an asexually produced progeny round about, 

 and in asexual continuity with itself. But they do not always remain 

 continuous; the hydra produces buds, but eventually sets them adrift. 

 This is still better seen in many of the hydroids, where individuals are 

 separated off as swimming-bells or medusoids. The multiplication 

 has become discontinuous. Continue the process, and we find the 

 liberation of special cells, clinging 

 often for a time to the parent, gen- 

 erally dependent for development on 

 union with similar cells of comple- 

 mentary constitution; we find, in 

 fact, the sexual reproduction which, 

 in the higher organisms, so thor- 

 oughly replaces the asexual process. 



IV. Occurrence of Asexual 

 Reproduction in Plants and 

 Animals. — In plants, as one would 

 expect from their typical vegetative 

 constitution, the asexual process is 

 common, particularly among the 

 lower forms. The most familiar of 

 all cases is afforded by the common 

 liverworts {Marchantia and Lunu- 

 laria), which through the formation 

 of asexual buds or gemmae in the cups so familiar upon their thallus, 

 are enabled to overrun our flowerpots, and so rapidly become a pest 

 of the greenhouse. Many ferns, too, notably among the Asple?ihims, 



FlG. 53. — Asexual Propagation of Grass — (a) 

 the bulbils rooting on the ground ; {b) 

 their appearance in the inflorescence: (c) 

 a small portion enlarged. — From nature. 



