576 A TEXTBOOK OF THEORETICAL BOTANY 



Development of the Embryo 



The antherozoids make their way down the neck of the archegonium 

 and one fuses with the oosphere. The resulting zygote then begins to develop, 

 but the process is remarkably slow and the young embryo, instead of growing 

 out towards the neck of the archegonium, pushes its way down into the 

 prothallus in the course of development. The first division of the oosphere 

 is transverse and separates an upper cell, the suspensor, which does not 

 divide again. The lower cell divides three times, forming an octant or 

 spherical group of eight cells. Two of the lower cells in this octant form the 

 stem and two form the first leaf, expanding obliquely upwards as they 

 develop. The four upper cells of the octant enlarge obliquely downwards 

 to produce a tuberous mass of cells called the foot, which remains as an 

 intraprothallial haustorium, deriving food from the prothallus until the 

 embryo at length becomes independent (Fig. 586). The first root arises 

 exogenously near the base of the first leaf, but its position is variable. Sub- 

 sequent roots develop endogenously. As the stem grows it emerges from 

 the prothallus and more leaves are formed. These first leaves have no mid- 

 rib and no vascular supply. They are sometimes called prophylls, and it 

 is interesting to note that in the small allied genus Phy/loglossum, from 

 Australia, all the leaves, even in the mature plant, are in this condition. It 

 has therefore been called " permanently embryonic." There is no mycorrhizal 

 Fungus in the cells of the embryo. The primary stem lasts only for a short 

 time and is then replaced by an adventitious outgrowth from its base which 

 becomes the horizontal stem of the mature plant. 



Eventually the prothallus decays and the young embryo develops more 

 roots and becomes entirely self-supporting. Its first vascular system is a 

 solid protostele, and the mature stelar arrangement is developed by a process 

 of furrowing, the phloem entering more and more deeply into the central 

 xylem mass and dividing it into separate plates. The whole process of the 

 embryological development is extraordinarily slow, and several years may 

 elapse before the young sporophyte reaches the surface of the soil. 



Alternation of Generations 



It will be seen that the life history of Lycopodium does not differ in any 

 essential way from that of the Ferns. Only in the relative importance of the 

 gametophyte, which has become modified in response to an underground 

 life do we see any marked difference. There is the same development of the 

 sporophyte as an independent structure, deriving its food during its young 

 stage from the prothallus and ultimately becoming self-supporting. The 

 development of the sex organs and the method of fertilization are not only 

 similar to those processes in the Ferns, but also show some relationship with 

 those in the Bryophyta, so that we see how, in spite of superficial differences, 

 we can compare the gametophytes of the two groups, and we realize that it 

 is correct to regard the prothallus of the Pteridophyta as homologous with 

 the gametophyte of the Moss or Liverwort. 



