470 HETEROMORPHISM AND ALTERNATION OF GENERATIONS. 



morphism; it will be seen in the sequel that Heteromorphism is a condition of 

 wide occurrence amongst plants. 



Zoophytes propagate themselves in two ways. They may produce buds which 

 grow into new individuals, just as buds arise on the branches of a tree and grow 

 into new branches; and, like the latter, the products of these buds remain attached 

 to the part of the colony producing them, so that ultimately the extent of the 

 colony is considerably augmented. In many Zoophytes, especially in the Polypo- 

 medusae, certain branches of the non-sexual polyp-form assume the form of cups 

 or capsule-like structures in which buds arise which grow into disc-like, free- 

 swimming medusae, with a crown of tentacles. These medusse contain sexual 

 organs, and from each of their fertilized eggs an embryo arises which becomes 

 attached to the sea-bottom, and grows either into a non-sexual polyp or into a group 

 of sexual medusse. In the last-named event the pear-shaped embryo, after swim- 

 ming about for a while, becomes attached by its pointed end. On its body arise a 

 number of ring-like furrows, which gradually deepen until the cone-shaped embrj^'o 

 is segmented into a number of transverse discs. Ultimately the cone disarticulates 

 and the discs swim away as medusse. These medusse are sexual persons, and from 

 the fertilized egg-cells, either sexual or non-sexual generations may arise. This 

 alternation of sexual and non-sexual persons is known as Alternation of Generations. 



Thus within the limits of the Zoophytes we see displayed two entirely distinct 

 things. First, heteromorphism, which gives us equivalent polyps on the same 

 colony, variously modified for the discharge of difierent functions; secondly, 

 alternation of generations, in which medusa-forms (sexual persons) arise by a process 

 of budding from polyp-forms (asexual persons), and give rise, by a sexual process, 

 to fresh polyps. Alternation of generations is an alternation of sexual and asexual 

 individuals, the one giving rise to the other. 



Both these phenomena are widely manifested amongst plants. The plant-bodj^ 

 amongst the Flowering Plants may be regarded as an assemblage of shoots. Each 

 shoot or branch-system consists of a series of members, of which the upper and 

 younger ones are developed with the assistance and co-operation of the lower and 

 older. These shoots are all united together, and the tissue which unites them,, 

 with its conducting-tubes and air-lacunse, may be regarded as an organ common 

 to them all. From the fact that the several shoot-members have the capacity 

 of independent existence, when separated from one another, they have been re- 

 garded as individuals and termed " Anaphytes " (c/. p. 6). Shoots, united together 

 into a plant-body, possess a common household, and division of labour is manifest 

 amongst them. The Anaphytes of the foliage region serve especially for the 

 preparation of food-stufFs, those of the flowering region for the production of 

 sexual cells and fruits. Shoots of the latter kind are termed flowers, of the former 

 foliage-shoots. Shoots arise from buds, and these may be similarly distinguished 

 into flower-buds and foHage-buds. Those which arise from foliage-buds remain, 

 for the most part, attached to the plant-body, appearing as branches of the same; 

 those, on the other hand, which arise from flower-buds ultimately disarticulate. 



