22 



QUEEN'S QUARTERLY. 



anatomy found associated with austere conditions f^-^J^.d, re 



treat is impossible. The upper or most '^^^'^'''^'^''^'''I'^lJZ 

 quently thickened and hardened, and often a ray filter, cons.st.ng of 

 layers of cells containing water, is formed between the exposed e,i,- 

 dermis and the mesophyll cells. The lower epidermis is also thick- 

 ened and the stomata are embedded in pits which protect them from 

 very rapid changes of atmosphere, such as accompany strong winds. 

 These modifications are characteristic of the leaves of our ever- 

 greens, as well as of those plants growing on open plains. In addi- 

 tion we find that the cells most active in the aljsorption of light and 

 in utilizing its energy for the manufacture of food for the plant, 

 have developed in prismatic form, and present their ends only to the 

 surface of the leaf, that is, to the rays of the sun. Because of their 

 elongate shape these cells together are known as palisade tissue, and 

 this is found in all but the leaves of extreme shade plants. Where 

 illumination is intense and drying winds frequent we find all these 

 anatomical modifications combined, and in addition other structures 

 to prevent the frequent change of atmosphere in contact with the 

 leaf cells. Vegetable hairs are common, and these are particularly 

 noticeable on the lower or sensitive side of leaves. These trichomes 

 are often of unusual shapes, such as branched hairs or star-shaped 

 or umbrella-shaped scales, usually forming a felt of greater or less 

 density. Being empty of all but air they serve both to reflect the 

 sun's rays and to hinder the escape of moist air. Our climate, — so 

 called temperate, but really reaching well into opposite extremes, — 

 gives us a flora which adapts itself to nearly optimum conditions in 

 summer, and to the extreme of drying cold in winter. 



In general we see that the action of protoplasm in this connec- 

 tion is what may well be expected from a sensitive substance of 

 limited powers of motion. Each peculiarity may be considered an 

 attempt at living along the line of least resistance, ^\'henever pos- 

 sible it retreats on receiving a threatening stimulus, and rebuilds the 

 outposts when favorable conditions return. In the great majorit}- of 

 cases this is more economical than to form defensive structures 

 capable of withstanding severe and prolonged attacks. When aus- 

 tere conditions are the rule,, however, and retreat means annihilation, 

 protoplasm becomes more tolerant of extreme conditions, and con- 

 structs defences behind which it can endure the unavoidable hard- 

 ships. 



W. T. MacClement. 



