ABSCISSION 649 



similar in all biennial species. Plants of this type develop only vegetatively 

 during their first growing season, producing underground organs which live 

 over winter. In many biennials the leaves are cold resistant and survive 

 the colder months of the year without injury. During their second grow- 

 ing season vegetative development is often renewed, but before long is largely 

 or entirely superseded by reproductive growth. Death of the plant follows 

 closely after the production of seeds and fruits. As with annuals the usual 

 life cycle of biennials can be modified by various circumstances. For example 

 many biennials become annuals when growing in warmer or longer-season 

 climates than those in which they normally behave as biennials. 



A greater diversity of cyclical patterns of reproductive and vegetative 

 development is found in perennial species than in those which live for only 

 one or two growing seasons. The following discussion refers primarily to 

 plants of temperate regions. In many woody perennials flowers are produced 

 in the spring before vegetative growth is resumed or concurrently with the 

 early stages in the development of the new leaf-bearing shoots. Examples 

 of species which exhibit this type of periodicity include many fruit trees 

 (peach, cherry, apple, etc.) and many forest tree species (elms, maples, oaks, 

 chestnut, Cottonwood, etc.). In some woody species such as the mulberry, 

 in which flowers develop from axillary meristems on the current season's 

 shoot, blooming occurs at the height of the season of vegetative growth. 

 Many woody perennials do not produce flowers until after the season's 

 vegetative growth is nearly or entirely completed. This is true of many 

 species which bear terminal inflorescences at the end of the current season's 

 shoots such as lilac, buckeye, and horse chestnut. 



As in woody species, flower production in herbaceous perennials may occur 

 before the vegetative growth of the same growing season, concurrently with 

 the production of stems and leaves, or only towards the end of a period of 

 vegetative development. The first of these types of growth periodicity, which 

 is the least common, is found in certain spring blooming species. The second 

 is also characteristic of many spring blooming herbaceous plants but is by no 

 means confined to such species. The third type of growth periodicity is 

 especially common among summer and fall blooming species, and is charac- 

 teristic of all species which produce terminal inflorescences on foliage-bearing 

 stems. 



Abscission. — Leaf-fall, particularly as it occurs from the stems of de- 

 ciduous trees and shrubs in the autumn, is a distinctive phenomenon of periodic 

 occurrence in plants. The abscission of leaves occurs at their point of attach- 

 ment to the stem. The phenomenon of leaf abscission is especially charac- 

 teristic of woody dicots, but also occurs in some herbaceous species such as 



