CORALLUMA, A NATIVE OF THE RED SEA REGION 

 This member of the Euphorbiaceje resembles a Cactus and has been guided in its development by the action of similar physical conditions 



SUGARS— PLANTS OF THE DESERT— SPINES 



D. T. MacDOUGAL 



Director Department of Botanical Research, Carnegie Institution of Washington 



Interpreting Recent Discoveries Concerning the Whys and Wherefores of the 

 Strange Habits of Some Cactus, Sedums and Other Succulents — The Modern 

 Understanding of What Has Hitherto Been Regarded as Defensive Armature 



Editors' Note: Formerly Assistant Director of the New York Botanical Garden, Dr. MacDougal came well equipped to his present work of direct- 

 ing botanical research, and the Desert Laboratory of the Carnegie Institution at Tucson, Arizona, affords ample opportunity for direct study of many 

 forms of desert plant life. Some interesting conclusions are presented in this article, the second of a series in which Dr. MacDougal discusses the 

 reactions of plants under varying conditions. "How Mountain Plants Behave When They Go to the Seaside," the first of the series, will be found on 

 pages 305-307 of The Garden Magazine for February. 



>UGAR as a substance potent in the evolution of plants 

 is a novel idea, but it is one to which the converging 

 researches of Professor Richards of Barnard College, 

 Dr. Spoehr of the Desert Laboratory, and my own have 

 brought us, and it may now be safely asserted that the differing 

 action of two kinds of sugar is the most important factor con- 

 cerned in the origin and evolutionary development of the 

 Cactus, the Live-for-evers, Sedums, and other succulents, as 

 well as the host of armed and spiny shrubs which live in the 

 drier regions of the world. 



This newly established conclusion may be best understood by 

 repeating some simple experiments which will illustrate the 

 fundamental qualities of sugars. 



Drop a cube of common sugar in a glass of water and watch 

 it dissolve slowly at first, then rapidly, especially if you raise 

 the glass and set the water swirling. A lump of salt would be- 

 have in the same way. Now put a large lump of cherry gum or 



gum arabic (acacia gum) in another glass, and you will see that 

 it does not break down and crumble as it goes into solution. 

 On the contrary it swells to many times its original size before 

 going into solution which it does with such difficulty that gen- 

 erally a mucilaginous residue remains. These differences of 

 behavior in the presence of water are characteristic of the two 

 types of sugar, for these gums are sugars to the chemist. 



Behead two carrots and scoop out a cavity as large as a 

 thimble in each one. Fill one with granulated sugar, and the 

 other with finely broken fragments of gum. A day later the 

 white sugar will be seen to have pulled water from the carrot 

 in which it has dissolved and may even overflow. The gum 

 may have become sticky where it touches the moist surfaces, 

 but it has pulled out almost no water from the intact cells. 

 This is in illustration of the fact that white sugar is osmotically 

 active and pulls water from other bodies with a force which is as 

 great as several atmospheres of pressure, and it may also pass 



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