834 A TEXTBOOK OF THEORETICAL BOTANY 



rest. (3) Propagation or vegetative spreading. (4) Climbing. (5) Photo- 

 synthesis, especially in cases where the leaves are short-lived or reduced. 



Variation associated with these functions leads to that great diversity of 

 form or habit, which is one of the most striking features of plant life. 



From the earliest times men have distinguished trees, shrubs and 

 herbs, and the distinction persisted even in scientific classification until the 

 eighteenth century. We shall deal with the classification of " Life Forms " 

 in Volume IV. Here we need only mention the distinction of habit which 

 divides plants into perennials, biennials and annuals. 



Perennials may be woody, either tree-like with one main trunk, or else 

 shrub-like with a cluster of stems. They may also be herbaceous, dying 

 down to the ground level each winter and persisting only by underground 

 organs. 



The duration of perennials is very variable and depends to some extent 

 on their rate of attaining flowering maturity. Some herbaceous plants live 

 only five to six years. Large trees, on the other hand, may take twenty-five 

 to thirty years to reach the flowering stage. The common factor in all 

 perennials is that they are polycarpic, that is to say, they floW'Cr and fruit 

 repeatedly. 



The age and size that trees may attain is not definitely limited. The 

 following are a few famous examples : — 



Eucalyptus aniygdalina. Australia. Height, 470 ft. Diameter at base, 



33 ft. Age, about 4,000 years. 

 Adansonia digitata. Africa. Diameter at base, 33 ft. Age about 5,000 



years. 

 Querais pediincidata. England. Diameter at base, 13-5 ft. Age about 



1,500 years. 

 Castanea sativa. Sicily. Diameter at base, 66 ft. Age about 700 years. 



Nevertheless trees are not immortal and there are historical records of 

 the apparently natural death of very old trees. 



Biennial plants last only for two years. In the first season they produce 

 at soil level a very contracted stem which bears a rosette of leaves. During 

 the second season the stem elongates and bears the flowers and fruit, after 

 which the whole plant dies. 



Annuals complete their whole life, from seed to fruit, in one year or less, 

 in some cases even in a few weeks, e.g., Senecio vulgaris (Groundsel), so that 

 several generations may be passed through in one summer, while nothing 

 but the seeds remains alive through the winter. Biennials and annuals are 

 therefore typically monocarpic, fruiting but once. 



Exceptions, however, occur. If an annual is prevented from ripening 

 its seed it may not die but may survive and flower again in a second or even 

 a third season. Certain monocotyledonous perennials, on the other hand, 

 are naturally monocarpic. Many species of Bamboo flower simultaneously 

 every twenty to thirty years and then die, leaving only seed to carry on the 

 race. The " Century Plant," Agave americana, reputed to flower only after 



