SOME PROTECTIVE DEVICES OF PLANTS. ig 



season of extreme drouth, for which perennial plants must prepare 

 as they do for the rainless season in tropical deserts. 



To meet the danger of seasons which favor excessive loss of 

 water, we find that plants have acquired certain habits which are 

 eminently important in the way of protection. One of the most 

 noticeable of these is the deciduous habit of most perennials of tem- 

 perate and subarctic regions. By this we mean the periodic loss of 

 all delicate and permeable tissue. The leaf is the chief organ for 

 transpiration of water, and the epidermis of the leaf is not suffi- 

 ciently waterproof to withstand the drying effect of either a hot rain- 

 less season or one of very low temperature, in which most of the 

 atmospheric moisture is precipitated as crystals of ice. Plants whose 

 aerial parts survive these seasons withdraw their protoplasm from 

 the leaves, the process being usually accompanied by marked changes 

 in color. Then the leaf separates at a plane of cleavage already pre- 

 pared, both surfaces of this plane being healed, leaving a scar but 

 not a wound. The parts of the plant which remain exposed to the 

 atmosphere are mostly encased in a coating of cork, which is suffi- 

 ciently impervious to prevent undue loss of water. There are, how- 

 ever, such delicate structures as the growing points, which require 

 special protection. They are usually encased in several layers of 

 embryonic leaves, which in turn are covered by firm, dense structures 

 which we call hud scales, and these may be coated with a waterproof 

 varnish. The purpose of these protective arrangements is not to 

 prevent the entrance of water, but its exit. 



Another seasonal habit is the retreat of many plants into sub- 

 terranean structures, such as storage roots and underground stems. 

 All parts left above ground perish, becoming dried straw, their living 

 contents having sought refuge beyond the reach of the drying at- 

 mosphere. Such plants as tlius secure the friendly protection of 

 mother earth are known as geophilous plants, and are largely repre- 

 sented among those which first in spring give us blossoms and leaves 

 as an earnest of the coming season. For this early and rapid growth 

 they are indebted to the stores accumulated during the previous 

 growing season, and guarded in some subterranean safe deposit. 



Perhaps the most remarkable preparation for an austere season 

 is that made by annual plants, — those which pass from seed to seed 

 in one season of growth. In these we have the plant dying com- 

 pletely, on the approach of unfavorable conditions, leaving only its 

 seeds to carry on the species. The seeds are small, compact parcels,, 

 each made up of a little plant, a store of food, and a strong overcoat. 



