THALLOPHYTES 



21 



15. Spore-reproduction. The second method of re- 

 production that appears among the Algae is reproduction by 

 spores. A spore is a special reproductive cell, as distinct 

 from a vegetative cell. For example, in such a form as 

 Ulothrix (Fig. 7), the vegetative body is a filament of cells 

 (A). These cells perform the ordinary vegetative work of 

 green cells when the con- 

 ditions favor such work ; 

 but if the conditions 

 change, they may begin 

 to form spores (B and 

 C) . The protoplast that 

 has been doing vegeta- 

 tive work divides, and 

 this division may be fol- 

 lowed by others, until 

 the wall of the old vege- 

 tative cell incloses a 

 number of new cells, 

 which are the spores. 

 The spores escape from 

 the old inclosure into 

 the water, and in Ulothrix 

 they swim freely about 

 by means of a tuft of 

 four cilia (hairs) at the 

 tip of each spore (C, 6). 

 These " swimming spores " are very characteristic of the 

 Algse, but the number and arrangement of the cilia vary. 

 For example, in (Edogonium (Fig. 15, A and C) the cilia 

 occur as a crown at one end ; in the brown seaweeds there 

 is a pair of cilia on one side of the spore (Fig. 16) ; in certain 

 forms there is a single cilium ; while the most common con- 

 dition is a pair of cilia at the apex of the spore (Fig. 8). 



It must not be supposed that spores are necessarily ciliated, 

 3 



FIG. 16. Ectocarpus : A, part of a filament 

 showing a sporangium distinct from the vege- 

 tative body, and also an escaped swimming 

 spore (enlarged) with its two lateral cilia ; B, 

 part of a filament showing a gametangium 

 distinct from the vegetative body, and also 

 an escaped gamete with its two lateral cilia. 



