FORMS OF VEGETATION 
i5° 
highway for migration. The general characters of Alpine 
plants are dealt with below. 
Forms of Vegetation. We shall now deal briefly 
with the morphology and natural history of a number of 
important for??is of vegetation. By this term we understand 
groups of plants, belonging to various natural orders, but 
presenting a general resemblance in their external habit, 
often correlated with resemblance in the conditions of life. 
According to the particular ecological character upon 
which we lay most stress, we may divide plants into groups 
in various ways, e.g. by differences in habit into trees, shrubs, 
and herbs; by different relations to water supply into hydro-, 
meso- and xero-phytes ; into erect plants, creepers, climbers, 
and epiphytes, and so on. 
Herbs 1 . According to their general habit plants may 
be roughly divided into trees , shrubs , and herbs ; the last are 
plants which do not possess any woody stem above ground, 
but are made up of softer tissues, and usually die down to 
the soil in autumn or after flowering. It is difficult to draw 
the line between herbs and shrubs ; such plants as the wall- 
flower, the base of whose stem is woody and persistent, may 
be termed suffruticose herbs. Many herbs are of great size, 
e.g. the agave, the banana (Musa), Amorphophallus, &c. 
All the Thallophyta and Bryophyta are herbaceous, and most 
existing Pteridophyta, though many fossil trees are known. 
In the Angiosperms the majority of species are herbaceous, 
and especially those of the higher orders (p. 120). 
Herbs may be annual , biemiial or perennial ; in the first 
case the entire life-history from germination to the ripening 
of the seed is carried out in one year. Some, e.g. Stellana 
7 nedia , go through several generations in one year, and may 
be termed ephemeral. In an annual plant there is no need 
for any storage of reserves except in the seeds ; the materials 
formed in the leaves are used directly in growth, flower- 
formation, &c. Correlated with this is the fact that the 
flowers are not as a rule very large or conspicuous (such 
flowers require much material for their formation) ; most 
1 Goebel, Organography of Plants ; Areschoug ( Geophile Pflanzen) 
in Acta Reg. Soc. Phys., Lund 1896; Vochting ( Knollengewcichse ) in 
Prings. Jahrb. f. wiss. Bot. 32, 1899; Freidenfelt ( Wnrzeln krautiger 
FJlanzen) in Flora, 91, 1902. 
