44 BOTANY 



roots, which are entirely absent. Also in many aquatics, Salvinia, 

 Wolffia arrhiza, Utricularia, Ceratophyllum, roots are altogether absent. 



The Ontogeny of Plants 



Just as in the phylogenetic development of the vegetable kingdom 

 there is an evolution from simpler to more complex forms, so each plant 

 in its ontogeny passes through a similar process of evolution. The 

 study of the ontogenetic development of a plant is termed embryology. 

 A young plant, in its rudimentary, still unformed condition, is called an 

 embryo or GERM ; and the early stages of its development are spoken of 

 as germination. As a rule, the embryo, in the beginning of its de- 

 velopment, is microscopic and of a spherical form. In a lower organism 

 this condition may continue from the beginning to the end of its 

 development, as is the case in Gloeocapsa polydermatica (Fig. 1, p. 11); 

 or the development may proceed further to the formation of filament- 

 ous, ribbon-like or cylindrical bodies. If the future plant is to have 

 a growing point, a part of the germ substance is retained in its 

 embryonic condition, and further development proceeds from this 

 embryonic substance. In the more highly-organised plants the different 

 members arising from the growing point only gradually attain that 

 degree of development characteristic of the particular plant. The 

 plant must develop and attain maturity, and it is not until it has 

 accomplished this that certain portions of the embryonic substance of 

 the growing point are appropriated to the production of new embryos. 



The different generations arising from an embryo of a plant may 

 exactly resemble each other, or an alternation of generations may 

 occur, in which case each succeeding generation is unlike its immediate 

 predecessor. As a general rule, the alternate generations are equiva- 

 lent, although this is not necessarily the case. One of the alternating 

 generations is usually sexually differentiated, that is, its reproductive 

 cells are only capable of development after a fusion with other repro- 

 ductive cells. This process of the fusion of two sexually differentiated 

 cells is called fertilisation, and its product a fertilised egg. The 

 asexual generation, on the contrary, produces reproductive cells, 

 termed spores, which require no fertilisation before germinating. In 

 the case of the Thallophytes, the alternation of generations is often 

 extremely complicated by the irregularity of the recurrence of the 

 different generations, and by the interposition of other modes of 

 reproduction, not in line with the regular succession of generations. 

 In the Cormophytes, however, asexual and sexual generations regularly 

 alternate, and consequently, whenever an alternation of generation 

 occurs, more than one generation is requisite to complete a cycle in 

 the development of a species. Accordingly, in the conception of a 

 species, two or more individuals are included. These individuals 

 may exist separately and distinct from each other, or they may be so 



