TWENTY-FIFTH ANNUAL MEETING. 95 



Its short, bunchy manner of growth and monotonoos gray tone are familiar to 

 every one who has been across the plains. 



Aristida purpurea Nutt., Bouteloua racemosa, B. hirsuta, B. oligostachya Torr., 

 Schedonnardus teianns Steud., and Munroa squarrosa Torr., are all very common 

 on the prairie, and many of them somewhat hairy or scabrous. 



There might be a great number of others added to this list, to illustrate the gen- 

 eral tendency of Western plants towards condensation of both leaf and stem, but 

 these amply illustrate this peculiar feature of the flora of a dry, arid region. 



The question that immediately occurs to the mind is: of what advantage are all 

 these peculiarities to a plant? Perhaps the first answer to occur to you is that in- 

 suflficient moisture and poor soil will naturally produce dwarfed or stunted plants. 

 Yet this does not explain either hairiness, or narrow foliage, or resinous coatings. 



There are five or six peculiarities to be explained in these condensed plants, all 

 interesting, and all explainable more or less satisfactorily. 



From the above comparisons of Eastern and Western species, the following pe- 

 culiarities have no doubt been noticed: Their reduced size; their hairy, wooly or resi- 

 nous covering; their rigidity and their finely-divided foliage; and the entire absence 

 of leaves on some of the plants. 



Aside from the dwarfing effect of drouth, there must be a reason why these pe- 

 culiarities are so strongly developed. Sir John Lubbock says every leaf form and 

 every special modification of any part has some special purpose or advantage for 

 the plant. 



This consolidation or condensation of plants, by shortening stems, thickening 

 leaves, or the substitution of fleshy, solid plants without foliage, of course reduces 

 the surface for evaporating moisture, and respiration and transpiration both take 

 place more slowly. Compare an ordinary prickly-pear cactus with any leafy weed, 

 like the Chenopodium, for instance, and note the vast difference between them in 

 the amount of surface exposed to sun and air for evaporation. 



This reduction of surface for evaporation or transpiration is the chief advantage 

 in the consolidation of the cacti in such a compact form, while the spines and 

 prickles protect them from the grazing animals that roam over this section devour- 

 ing every green herb. The rigidity and hardness of some other species probably 

 serve the same purpose. 



The hairy or resinous covering of leaves and stems forms protecting covers for 

 the stomata, preventing too rapid transpiration. 



The advantages of narrow or finely-divided foliage may be more than one, and 

 are so easily explained. Sir John Lubbock says, in his lecture on leaves, that large, 

 thin leaves are usually found in moist regions, protected from strong winds. Mrs. 

 W. A. Kellerman suggested this point in a paper ("The Evolution of Leaves") read 

 before the Academy several years ago. The point suggested by these two writers 

 gives us the clue to the explanation of the finely-divided foliage of this dry, windy 

 region. It has been previously stated that large, thin leaves are easily injured by 

 strong winds passing through them, while thinly-divided, rigid leaves would allow 

 the passage of the wind freely, without any injury to the foliage. The winds are 

 very strong and constant in all, this dry region; hence the need of the finely-divided 

 or linear foliage. 



There is still another means of reducing evaporation that occurred to me while 

 studying these Western plants. Might we not most naturally expect smaller sto- 

 mata, more deeply imbedded, and a small ernumber per square inch of surface, on 

 Western plants than on our Eastern species, growing in a more moist climate? 

 Thinking this most probably the case, I began immediately to measure and com- 



