322 BOTANY 



{JB): The gametes, which may be produced in large numbers (32 or 64) in every 

 cell (C), possess two delicate cilia, a red eye-spot, and a ehromatophore. After 

 swarming, the gametes conjugate in pairs (E) and give rise to zygotes {F). The 

 zygotes become invested with a thick wall, and serve as resting-spores (ff). While 

 the gametes of Sphaerella and of most other Volvocaceae are similar and of equal 

 size, in the case of Eudorina and Volvox, which may also be considered as the 

 most highly-developed forms of the whole order, the sexual cells are more differ- 

 entiated, and assume the form of large passive egg-cells and small biciliate sperma- 

 tozoids. The genus Volvox, as represented by the species V. globator and V. 

 aureus (V. minor), found in small pools and ditches, forms hollow, spherical 

 colonies (ccenobia), which are often large enough to be visible to the naked eye. 

 The colonies are composed of numerous cells (up to 22,000), regularly distributed 

 in a peripheral layer. The cells are connected laterally with each other by proto- 

 plasmic threads, usually six in number, which extend through their distended cell 

 walls (Fig. 242, A), and from each cell two delicate cilia are given off externally. 

 The Volvox colonies multiply vegetatively by the formation and final escape of new 

 daughter colonies, resulting from the division of a single cell (A, t). Spermatozoids 

 and egg-cells are produced either in the same or different colonies. The spermato- 

 zoids arise through the division of special cells (so-called antheridia) into numerous 

 daughter cells, which eventually form tabular packets of elongated spermatozoids 

 (B, C). The anterior extremity of the spermatozoids of Volvox aureus is colourless, 

 and terminates in two cilia ; in their opposite, posterior end the spermatozoids 

 contain a bright green ehromatophore. In the anterior portion there are a lateral 

 red eye-spot, two contractile vacuoles, and a cell-nucleus (D). The egg-cells are 

 produced by the enlargement of individual cells of the colony. They are large 

 and green, non-motile, and surrounded by a gelatinous envelope (A, o). After 

 fertilisation by the spermatozoids, which, in swarming, escape into the interior of 

 the hollow spherical colony, they become transformed into firm -walled resting 

 oospores, which on germination gives rise to a new colony. The mother colony 

 dies after the egg-cells have reached maturity. 



Order 2. Confervoideae 



The Confervoideae exhibit, as compared with the unicellular Proto- 

 coccoideae, an advance in the external segmentation of the thallus. It 

 is always multicellular, and, in most of the genera, consists of simple 

 or branched filaments. The thallus of the marine genus Ulva ( Ulva 

 laduca, SEA lettuce) has, however, the form of a large, leaf-like cell 

 surface (Fig. 5, p. 1 2). Although a greater part of the Confervoideae live 

 in fresh or salt water, where they are found either free-swimming or 

 attached to some substratum by a colourless basal root-cell, a few 

 aerial forms (Chroolepideae) grow on stones, trunks of trees, and, in the 

 tropics, on leav.es. To this family belongs the aerial Alga Trentepohliu 

 (or Chroolepus) Jolithus, often found growing on stones in mountainous 

 regions. The cell filaments of this species appear red on account of 

 the hsematochrome they contain, and possess a violet-like odour. 



The asexual reproduction of the Confervoideae is accomplished by 

 the formation of ciliated swarm-spores, although in many cases they 

 may also develop resistant resting-spores. 



