THE ORIGIN OF SEX IN PLANTS 15 



The immediate effect of such simple syngamy as this 

 is a reduction in number of cells from two to one. A 

 consequence of it is an opportunity for the pooling of 

 the heritable qualities of two cells, which have been of 

 slightly different origin. These are the fundamental 

 lessons taught by Ulothrix, a plant in which the sexes 

 are not distinguishable from one another. 



Whether in Animals or Plants, it is only in the simplest 

 cases that the distinction between the two sexes is absent. 

 There is evidence from both sources that a difference 

 between the sexes has been acquired by gradual steps. 

 In no group of Plants is this more clearly shown than 

 in the Brown Seaweeds, the best known of which is the 

 Common Bladder Wrack (Fucus vesiculosus) , present on 

 all shores. This plant, however, with its broad leathery 

 fronds, is itself an advanced type of its class. Some of 

 the Brown Seaweeds are simple and filamentous ; and 

 it is among these that the most primitive conditions of 

 sex are found. In fact, complexity of structure and 

 elaboration of sex run parallel in them. For instance, 

 in Ectocarpus siliculosus, which consists of delicate 

 filaments, partitioned sporangia are found at the ends of 

 the branches, which open when ripe, and allow their 

 contents to escape into the sea-water as motile gametes 

 (Fig. 7). In form and size these are all alike, being 

 pear-shaped, with two cilia attached laterally ; while 

 within the protoplasm is a nucleus, and a red eye-spot. 

 But though the sexual cells are thus isogametes as regards 

 form, those from distinct sporangia differ in their behaviour, 

 so that they may be ranked functionally either as male 

 or female. For it is found that the gametes from certain 

 sporangia are at first motile, but soon lose their motility, 

 and attach themselves to some solid substratum. These 



