66 SWIMMING SPORES. EYE-SPOT. [chap. 



In other cases the dispersion is mainly the work of 

 the seed itself. In some of the lower plants, as, for 

 instance, in many seaweeds, and in some allied fresh- 

 water plants, such as Vauckeria, the spores^ are 

 covered by vibratile cilia, and actually swim about 

 in the water, like infusoria, till they have found a 

 suitable spot on which to grow. Nay, so much do 

 the spores of some seaweeds resemble animals, that 

 they are provided with a red " eye-spot " as it has 

 been called, which, at any rate, seems so far to 

 deserve the name that it appears to be sensitive to 

 light. This mode of progression is, however, only 

 suitable to water plants. One group of small, low- 

 organised plants, Marchantia, develop among the 

 spores a number of cells with spirally thickened walls, 

 which, by their contractility, are supposed to dis- 

 seminate the spores. In the common Horsetails 

 (Equisetum), again, the spores are provided with 

 curious filaments, terminating in expansions, and 

 known as " elaters.'' These move with great vigour, 

 and probably serve the same purpose. 



In much more numerous cases, seeds are carried by 

 the wind. For this of course it is desirable that they 

 .should be light. Sometimes this object is attained 

 by the character of the tissues themselves, sometimes 

 by the presence of empty spaces. Thus, in Valerianella 

 auricula, the fruit contains three cells, each of which 

 would naturally be expected to contain a seed. One 

 seed only, however, is developed, but, as may be seen 

 from the figure given in Mr. Bentham's excellent 



^ I need hardly observe that, botanically, these are not true seeds, 

 but rather motile buds. 



