3i6 SENESCENCE AND REJUVENESCENCE 



THE ORIGIN OF THE GAMETES IN PLANTS 



Thus far no evidence has been discovered among the plants of an 

 early separation of the primitive germ cells from other so-called 

 somatic portions of the organism, such as has been described for 

 various animals (see pp. 323-33). No Keimhahn or germ path 

 exists in the plants, that is, the germ cells cannot be followed through 

 the developmental history as cells or protoplasmic regions distinct 

 from other parts of the body. 



In that group of the green algae known as the Conjugales, 

 which includes Spirogyra and the desmids, in the diatoms, and 

 in most of the ciliate infusoria among the animals, the cell which 

 constitutes the body of the organism becomes the gamete without 

 any or with comparatively little visible structural change; two 

 such cells conjugate, and their contents fuse to form the 

 zygospore. 



In other algae and in those fungi in which gametic reproduction 

 is known to occur, the gametes are always more or less different 

 both in morphological structure and behavior from other parts of 

 the organism, but they originate from the plant body and to all 

 appearances are the most highly specialized parts of the species, 

 and, finally, in most cases, show a high degree of sexual differentia- 

 tion, as the following figures show. Fig. 123 shows the young egg 

 cell of Volvox, Fig. 124 a Volvox spermatozoid. Fig. 125 the oogonium 

 and antheridium of the alga Oedogoniimi with female and male 

 gametes, Fig. 126 the sex organs of Chara with the single egg in the 

 oogonium, and Fig. 127 a spermatozoid of Chara. In Fig. 128 

 the sex organs of the fungus Saprolegnia and their relation to the 

 vegetative part of the plant are shown. In all these cases 

 the gametes show the same sort of sexual differentiation as in the 

 multicellular animals. In the mold Mucor, however, the ends of 

 two hyphae enlarge and come together, and a gametic cell is sepa- 

 rated from each (Fig. 129), but the two cells are not, so far as 

 known, sexually differentiated. These two cells increase in size 

 (Fig. 130) and unite to form the zygospore (Fig. 131). In none 

 of these cases is there any trace of an early segregation of the germ 

 cells from the rest of the plant. The sex organs and germ cells 

 appear only when the plant attains a certain physiological condition. 



