Dispersal of Herbaceous A ngiosperms. 595 
of parenchyma until typically herbaceous stems have been produced. 
A large proportion of the species counted as herbs in our tabulations of 
tropical floras is made up of such scandent types, which are essentially 
different from the terrestrial herbs characteristic of temperate regions. 
Any xerophytic condition tends to dwarf plants and, if it is periodic, 
to produce herbaceous forms. Desert vegetation is composed largely of 
shrubs or undershrubs, but there is also a considerable body of herbs, 
both annuals and perennials, which spring up rapidly and produce seed 
during any rainy period, and which have the same relations to extremes of 
moisture that ordinary herbs have to extremes of temperature. Many 
of the endemic herbs of South Africa, West Australia, and other regions 
which are subject to dry seasons of considerable length, have in all 
probability been developed in this way. 
It is altogether probable that dicotyledonous herbs were developed 
in rare instances in the Cretaceous or very early Tertiary, before extremes 
of climate had been generally produced and in response to local xerophytic 
conditions, probably for the most part in the mountains ; but it seems very 
probable that the great mass of herbaceous vegetation, certainly among the 
Dicotyledons, is of much more recent origin. 
VI. General Conclusions. 
Let us now enumerate briefly some of the general conclusions at which 
we have arrived as to the origin and dispersal of herbaceous Angiosperms. 
That the herbaceous vegetation of to-day should be regarded as 
of comparatively recent development is indicated by evidence from Palaeo- 
botany, that almost all the earliest Angiosperms of which we have knowledge 
were woody ; from Phylogeny, that herbs tend to occur more frequently in 
plant-groups which are recent rather than in those which are primitive ; from 
Anatomy, that the ancient type of cambial activity produced a solid ring of 
secondary wood, and that the herbaceous stem has been reduced from such 
a condition ; and finally from Phytogeography, that herbaceous vegetation, 
most richly developed in the great land-mass of the north temperate zone, 
has spread thence over most of the globe ; and that regions which have 
long been isolated consequently possess a flora which, especially in its 
endemic portion, is very ancient and therefore composed almost entirely of 
woody plants. The progressively increasing refrigeration of the climate of 
the north temperate zone has been by far the most important (though not 
the only) factor in the development of herbs; for these plants,- from the 
brevity of their life-cycle and their ability to survive periods of extreme cold 
either underground or in the form of resistant seeds, are able to thrive 
in regions which are subject to winters severe enough to eliminate most 
plants with perennial aerial stems. Herbaceous plants, having in this way 
