200 



STRUCTURAL BOTANY 



cell- walls follow, and soon the oospore (which here does 

 not pass through a resting-stage) is converted into a little 

 mass of tissue, but without at first changing its external 

 form. After eight or ten days, several root-hairs begin 

 to grow out at the end away from the light (see Fig. 

 86, B). They burst through the oogonial wall, which 

 has lasted all this time, and attach the embryo to the 



rock or whatever 

 else it may be lying 

 upon. The upper 

 part of the embryo 

 now elongates and 

 becomes first 

 cylindrical and 

 then flattened at 

 the free end ; a 

 depression soon 

 arises at its apex, 

 in which a definite 

 apical cell appears, 

 and now we have 

 in all essentials a 

 new Pelvetia plant 

 fairly started in 

 the world. 



In these Algae 

 the result of fertilisation is a plant just like the parent. 

 There is no kind of asexual reproduction, and therefore 

 no possibility of any alternation of generations. 



We see, then, that in the order Fucaceaa we 

 have the simplest possible life -history combined with 

 a very perfect form of sexual reproduction. The 

 plants are altogether very highly organised, as shown 



FIG. 87. Ovum of one of the Fucacese (Asco- 

 phyllum nodosum), seen in section at the 

 moment of fertilisation. , small male 

 nucleus of a spermatozoid, which has traversed 

 the protoplasm, and is now in contact with 

 the large nucleus of the ovum. The proto- 

 plasm of the ovum shows a distinct foam-like 

 structure. Magnified about 650. (After 

 Farmer.) 



