462 A TEXTBOOK OF GENERAL BOTANY 
some cases the aérating roots are -slender structures which grow 
vertically out of the soil, while in still other cases they have the 
form of knees. 7 
Another characteristic of a large proportion of mangrove-swamp 
trees is the germination of the seeds while still within the fruits. 
This feature is most strikingly shown inthe family RAzophoraceae, 
in which the seedling bursts through the fruit and hangs down 
as a long, slender structure while the fruit is still attached to 
the tree (Fig. 300). Finally the seedling drops from the fruit, 
sticks in the mud, and continues to grow (Fig. 301), or it may 
be transported by water and deposited in a situation that is 
favorable to growth. : 
Succession and climax vegetation. The large vegetational re- 
gions, such as rain forest, deciduous forest, deserts, and arctic 
tundra, are determined by climate, the principal factors being 
moisture and temperature. In all extensive regions there are, 
however, minor areas in which the vegetation is the result not 
of the general climatic complex but of purely local conditions 
such as the nature of the substratum or the interference of man. 
Vegetation that is in a stable condition and represents the highest 
type that the climate can support is climax vegetation. Before 
vegetation reaches a climax, or stable condition, it may pass 
through many successive stages that are influenced by the local 
environment. ‘The giving way of one type to another is known 
as succession. 
In a region in which tall forest is the climax type there may 
be a lake which results in the presence of several local types 
of vegetation. The vegetation in the deepest part of the lake 
in which there are rooted plants may consist entirely of sub- 
merged plants, and between these and the forest the following 
types of vegetation may occur in successively shallower water: 
plants with floating leaves, erect monocotyledons projecting out 
of the water, and a growth of shrubs. The level of the water 
in lakes does not remain constant; on the contrary, the depth 
of the water tends to become less and less. This is due to the 
fact that material is constantly being deposited in the lake and so 
