202 EXTERNAL CONFORMATION OF PLANTS. 



same manner the whole (branched) leaf-blades of many Acacias are also arrested, and are 

 replaced by the petiole (phyllode), which is then expanded in its median plane. Still 

 more complete is the arrest of the leaves from the axils of which spring the branches of 

 the panicles of Grasses ; and in this class whole flowers are often frequently aborted. In 

 diclinous Phanerogams the unisexuality of the flowers usually depends on the abortion of 

 the stamens in the female, of the carpels in the male flowers. Sometimes only one of 

 several stamens is aborted, as in Gesneraceae {e. g. Columnea, where it is transformed 

 into a small nectary); and the same occurs with the carpellary leaves {e.g. in Tere- 

 binthacese). In all these cases the structure which is afterwards arrested is actually 

 present in the bud or even later, but its further growth then ceases. The comparison, 

 however, of nearly related plants shows that very commonly certain members are want- 

 ing in the flower the presence of which might be expected from the position and num- 

 ber of the others and from their presence in nearly related forms, although in such cases 

 even the earliest condition of the bud does not show the absent member. Since from 

 the point of view of the Theory of Descent it must be assumed that nearly related plants 

 are descended from a common ancestral form, the absent member may in such cases be 

 also supposed to be aborted, only the arrest of development w^iich has once taken place 

 at an early period is so complete and has become so hereditary, that even its first 

 rudiment is suppressed. The true theory of the structure of many flowers, and the 

 reference of different forms of flowers to common types, often depends on the restoration 

 of aborted members of this kind ; but to this we shall recur in detail in the Second Book, 

 when treating of Phanerogams. 



Sect. 29. Alternation of Generations. — When the growth of a plant has 

 continued for some time, so that it has assumed a certain definite internal and 

 external differentiation and conformation, a period at length arrives at which shigle 

 cells become detached from the organic connexion, and cease to participate in the 

 growth which they have hitherto shared as integral parts of the plant which has 

 produced them. These cells begin, either immediately or after further preparation, 

 an independent course of development. A structure thus arises which can no 

 longer be considered a part of the connected conformation of the mother-plant, 

 but a new plant which may be like or unlike the one which produced it. 



Cells of this character which are separated from the organic structure of a 

 plant, even if they do not always abandon the place where they were formed, are 

 Reproductive Cells ; and those plant-structures which result from similar reproductive 

 cells, and are also like one another, form a Generation. 



But it is only in some Algae and Fungi that (according to the present state of 

 our knowledge) all successive generations are similar and produce similar repro- 

 ductive cells {e. g. in Nostocacese, Spirogyra, &c.). Even in most Thallophytes 

 and in all Muscinese and Vascular plants the generations which proceed from 

 one another are dissimilar^ or produce dissimilar kinds of reproductive cells, from 

 which plant-structures of dissimilar habits of life and dissimilar conformation arise. 

 In such cases several similar generations (A, A, A, &c.) may first of all appear in 

 succession, the last of which brings forth a dissimilar generation (B), which, on its 

 part, again produces a generation of the first kind (A), as, for instance, occurs in 

 Vaucheria and Saprolegnia; or three or even four dissimilar forms of generation 

 (A, B, C) succeed one another, until at last the first form (A) again appears. 

 One form may repeat itself again several times (A, B, B, B, &c., C, A) before 

 the production of a new one. This is the case in the Hypodermias among Fungi. 



