336 



BOTANY OF THE LIVING PLANT 



each, in Pelvetia two, and in Himanthalia and others only one. Vestigia 

 of the atrophied eggs are found, which clearly indicate that their number 

 has been reduced. Thus the Brown Algae form a coherent series of sexual 

 differentiation. Their sexual cells probably originated from motile cells 

 all alike. The first functional though not formal distinction of sex is seen in 

 Ectocarpus siliculosus (Fig 285). Steps in loss of motility are seen in other 

 species, and in Cutleria (Fig. 286). while in Fucus the large ovum is entirely non- 

 motile (Fig. 287). Finally, in Himanthalia a single ovum occupies the whole 

 oogonium : its large size giving such security of germination as to justify the 

 reduction in number to one. The whole series thus illustrates steps in sexual 

 differentiation, which are biologically intelligible. 



Fig. 288. 



Fucus A, oogonium, the contents of which have divided into eight eggs. £ = 

 oogonium, from which tbe contents (C) have been extruded. D, E, liberation of 

 the eight eggs ; st = stalk ; mes = middle — end = inner layers of the oogonial wall. 

 (After Thuret.) (From Strasburger.j 



In Fucoids which live submerged the gametes may be set free at 

 any time. But when exposed between tide-marks it will be chiefly 

 on the rising tide that they are liberated. Exposed to the air the 

 mucous thallus shrinks on drying, and the pressure of the contracting 

 tissue may be seen to squeeze out the ripe antheridia and oogonia, 

 which may be recognised by their yellow or olive colour, through the 

 open pores of the conceptacles. If these be collected fresh in separate 

 watch-glasses in sea water, the final liberation of the minute motile 

 spermatozoids (Fig. 287, 4) and of the much larger non-motile eggs 

 (Fig. 288, E) can be easily followed. If then a drop of water containing 

 the former be added to a drop containing ova on a slide, many sperma- 

 tozoids will be seen to collect round each ovum, which thus shows 



