6 THE STORY OF PLANT LIFE 
and moisture. These factors being vital to plants, 
the seasons greatly influence plant-life. 
Spring, generally speaking, is the period when life 
is renewed, when activity is stimulated, when the 
plants that prolong their life beyond the single year, 
or two years, and are perennial, begin to put forth 
again those aérial parts which are of so much 
importance in the nutrition, growth, with attendant 
assimilation, respiration, transpiration, etc., and 
reproduction of plants. Plants that are evergreen 
remain inactive during the winter to a great extent, 
transpiration being difficult. Deciduous trees and 
shrubs put forth anew their buds, and the shoots 
break forth into leaf, so that the carbohydrates 
derived from the carbon dioxide in the air by the 
action of sunlight upon the chlorophyll may contribute 
their part towards the well-being of the plant. 
In the tree the sap begins to rise, and a fresh zone 
of wood is commenced by addition to the exterior in 
the Dicotyledons, to the interior in the Monocoty- 
ledons. Aquatic plants begin to struggle upward 
from the river or lake-bed towards the surface. 
The sere grass begins to assume a green and fresh 
appearance. Already the bulbous plants that have 
stored up their reserves during the last year have 
begun before winter is over to put forth the leaves 
and bloom, so that they may flower in the woods 
before the thick cover of leaves shuts out the sun. 
The summer months also have their part in the 
general advance of the vegetation of each season. 
