HISTORICAL. 5 



torical side of the question of the acidity of succulents and its significance from 

 all points of view. As a result of his own experimental work he decided that 

 the acid formation and its periodicity was peculiarly characteristic of plants 

 which, by reason of their protection against high transpiration-rate, are not 

 favorably placed as to gas-interchange relations. He concluded that the 

 breaking up of the acid affords an important saving of carbon dioxide, since 

 this takes place in the daytime when the liberated carbon dioxide will be re- 

 absorbed in the photosynthetic processes. This is essentially the point of 

 view taken in Pfeffer's" digest of the matter, and is the opinion that holds 

 generally. The acid collects at night as a result of respiratory processes, and 

 this acid when it degenerates during the daytime affords a source of carbon 

 dioxide which is utilized in photosynthesis. 



Aubert 6 has published several important papers. He thought that in the 

 cacti it is malic acid proper that is produced, but that in the Crassulaceae 

 the substance is somewhat different and is to be regarded as isomalic acid. 

 He corroborated the general facts already known and accepted, and he ex- 

 tended knowledge of the questions involved by a careful investigation of the 



f^f\ 



^^ quotient. The normal gas-ratio in succulents may be regarded as less 



than unity. The more succulent a plant is, the more acid it contains, and 

 hence it will absorb in the dark an increasing amount of oxygen with a mini=* 

 mum output of carbon dioxide, as a consequence of which its gas-interchange 

 ratio falls. The amount of carbon dioxide given off increases with rising 

 temperature, or with long-continued darkness, as well as with diminishing 

 succulence, hence under these conditions the ratio rises. He argued that suc- 

 culents produce malic acid at the cost of liberation of carbon dioxide, and when 

 with higher temperature or other causes the formation of malic acid is inhibited 

 more of this gas is given off in proportion to the oxygen absorbed, so that the 

 amounts of the two become more nearly equal. Not only do the succulent 

 plants present an unfavorable epidermal and stomatal provision for gas 

 interchange, but the succulence itself inhibits free passage of gases into and out 

 from the tissues and hence the peculiar phenomena of their gas-ratio is cor- 

 related with their anatomical structure, which in turn is a result of their 

 habitat. 



A very comprehensive examination of the whole question of plant acidity 

 has been made by Purjewicz, c covering the problems of the formation and 

 destruction of organic acids in the higher plants in general. His results as to 

 the sources of decrease in acidity confirm those of de Vries and other workers, 

 namely, that the causes are light, continued darkness, and high temperatures. 

 He regarded the breaking down of the acids as an oxidation process and 

 pointed out that in the absence of oxygen this process is greatly inhibited. 

 He adopted the already accepted conclusion that the carbon dioxide formed 

 by the degeneration of the malic acid in light furnishes material for photo- 

 synthesis, and that its relation to the latter process is only indirect. That 

 there is any increase in the carbohydrates accompanying the decrease in 



Pfeffer, Wm. Physiology of plants (trans, by Ewart), vol. 1, p. 326, 1904. 



"Aubert, E. Sur la repartition des acides organiques chez les plantes grasses, Rev. Gen. 

 Bot., vol. 2, p. 309, 1890. Recherches physiologiques sur les plantes grasses, Ann., Sci. Nat. 

 Bot., Series 7, vol. 16, p. 1, 1892; also as a dissertation in 1892. 



<An elaborate abstract in Botanisches Centralblatt, vol. 58, pp. 368-374, Kiew, 1893 (Russian). 



