20 QUEEN'S QUARTERLY. 



■With this outfit they are confidently expected to prevent the family 

 from becoming extinct. We have marked evidences of their success, 

 in spite of the fact that man, in common with most of the other ani- 

 mals, takes advantage of their youth and helplessness to rob them of 

 their stores of starch, oil, or proteid matter. In this and other ways 

 the vast majority of seeds are destroyed, but usually sufficient of 

 them escape to carry on the species. The deciduous, the geophi- 

 lous, and the annual habits above described may be considered as 

 extreme cases of reduction of exposed surface, for a season during 

 which growth is impossible and destruction by drying quite certain. 

 The biennial habit is quite similar. Here two short growing seasons 

 are required for the plant to pass from seed to seed. During the 

 first of these a quantity of food is formed and stored in a protected 

 position, usually but not always underground. During the second 

 favorable season a flowering stem, bearing inconspicuous leaves but 

 plenty of flowers, is sent up at the expense of the foodstore. At the 

 end of this season the plant dies just as annuals do, leaving only 

 seeds for the perpetuation of its kind. 



There are many situations in which water is scanty during the 

 whole growing season, while the heat is so great as to make evapo- 

 ration a serious menace to the life of the plant. Under such circum- 

 stances we find the protoplasm protecting itself by various modifica- 

 tions of the methods above described. In general there is a reduc- 

 tion of the delicate tissues through which water may be lost. Plants 

 growing on dry plains and sand dunes show what we call a stunted 

 condition. A closer examination shows rather that the habit of the 

 plant is more compact than in favorable situations, and that the 

 leaves, while of less surface, have greater thickness and are of 

 firmer texture. This reduction of evaporating surface becomes more 

 marked as we study plants exposed to increasingly severe and con- 

 tinuous drouth, until the reduction of leaves culminates in cylindrical 

 and finally spherical plants with no leaves whatever. These are the 

 cacti of the deserts, having the greatest mass for the least surface 

 and with the work ustially performed by the leaves transferred to 

 the green stems. 



From changes involving the whole aerial plant and relating to 

 seasons in whicli growth or even the maintenance of life is i 

 impossible, we now turn to the effect of temporary conditions W 1 

 would conduce to harmful evaporation. Here we shall find H' 

 leaves modified to a greater or less degree. To understand th ^ 

 fications we must first note the structure of foliage leaves ^C '" 



