Page 28 - Algae

General Overview 11

FIGURE 1.14 Siphonous thallus of Vaucheria FIGURE 1.15 Pseudoparenchymatous thallus
sessilis. of Palmaria palmata.

Statospores are endogenous cysts formed within the vegetative cell by members of Chrysophy-
ceae such as Ochromonas spp. The cyst walls consist predominantly of silica and so are often pre-
served as fossils. These statospores are spherical or ellipsoidal, often ornamented with spines or
other projections. The wall is pierced by a pore, sealed by an unsilicified bung, and within the
cyst lie a nucleus, chloroplasts, and abundant reserve material. After a period of dormancy the
cyst germinates and liberates its contents in the form of one to several flagellated cells.
Akinetes are of widespread occurrence in the blue-green and green algae. They are essentially
enlarged vegetative cells that develop a thickened wall in response to limiting environmental nutri-
ents or limiting light. Figure 1.18 shows the akinetes of Anabaena cylindrica (Cyanophyta). They
are extremely resistant to drying and freezing and function as a long-term anaerobic storage of the
genetic material of the species. Akinetes can remain in sediments for many years, enduring very
harsh conditions, and remain viable to assure the continuance of the species. When suitable con-
ditions for vegetative growth are restored, the akinete germinates into new vegetative cells.

SEXUAL REPRODUCTION
Gametes may be morphologically identical with vegetative cells or markedly differ from them,
depending on the algal group. The main difference is obviously the DNA content that is haploid
instead of diploid. Different combinations of gamete types are possible. In the case of isogamy,
gametes are both motile and indistinguishable. When the two gametes differ in size, we have het-
erogamy. This combination occurs in two types: anisogamy, where both gametes are motile, but
one is small (sperm) and the other is large (egg); oogamy, where only one gamete is motile
(sperm) and fuses with the other that is non-motile and very large (egg).