. A manual of botany. Botany. THALLOPHYTA—ALG^ 55 F:g. 803. a number of bi-ciliated gametes, which are liberated by rupture of the cell-wall. These, after a period of activity, conjugate in pairs, the conjugating cells generally originating from two different cells or even two different filaments. The differentia- tion of sexuality in some of these is very slight, for if they fail to conjugate they can still germinate as if asexual cells. In another group, while apparently any ceU of the filament can become a gametangium, the contents are not differentiated into oihated free-swimming bodies, b
. A manual of botany. Botany. THALLOPHYTA—ALG^ 55 F:g. 803. a number of bi-ciliated gametes, which are liberated by rupture of the cell-wall. These, after a period of activity, conjugate in pairs, the conjugating cells generally originating from two different cells or even two different filaments. The differentia- tion of sexuality in some of these is very slight, for if they fail to conjugate they can still germinate as if asexual cells. In another group, while apparently any ceU of the filament can become a gametangium, the contents are not differentiated into oihated free-swimming bodies, but the whole of the proto- plasm undergoes a kind of rejuvenescence and forms a single gamete. In other forms definite antheridia and oogonia are produced, which furnish antherozoids and oospheres respect- ively. In each oogonium, as a rule, only a smgle oosphere is produced, and this is fertilised in situ by the entry of an antherozoid into the oogonium [fig- 805). This marks a striking advance upon the sex- ual process in the Phseophyeese, where the oospheres are set free and fertihsed out- side in the water. It approaches to the con- dition in the next group, where more elaborate provision is made to bring the sexual cells together. In the cases of conjugation of the solitary gametes in a game- tangium, the fusion of the cells always occurs within one of the cells or an outgrowth from it, the gametes never being set free. In the latter case parthenogenesis sometimes occurs, a gamete developing into a new filament without any process of conjuga- tion. Such gametes are called azygospores. In one exceptional case, Sphceroplea, several oospheres are formed in the oogonium, which is not a specially differentiated cell. Here, too, fertihsation takes place internally, antherozoids finding their way into the cell by an opening in the wall. Asexual cells are produced by most, but not by all the mem- bers of this group. They are generally free-swimming cells, or in some cases ccenoc
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectbotany, bookyear1895
