168 Physiologie. 



cacti, the importance of these plants in various parts of Arizona 

 as food for cattle led to the outlining of these investigations. The 

 experiments have been greatly suggested through the failure ofthe 

 various Burbank spineless cacti to grow successfully under Ari- 

 zona conditions. 



The spineless cacti studied are: Opuntia castillae, 0. ficus in- 

 dica from Malta, O. ficus indica from Sicily, O. Ellisiana, O. fusi- 

 caulis and O. sp. Burbank Special. 



The anatomical structure of the cacti was first studied. The 

 cuticle of the epidermis varies greatly {Opuntia castillae 15—20 ,«, 

 O. spec. Burbank Special 5—8 p). Immediately below follows the 

 crystal-bearing layer, it usually contains crystals, and has practi- 

 cally the same thickness (in all species 35— 40ju). The next layer is 

 usually three to five cells thick and with age has very thick mem- 

 branes, its thickness varies very much (0. castillae 100—120 /u, O. 

 ficus indica from Malta 55—75 /i). The following tissue belongs to 

 a large celled and thin walled parenchyma. 



The first three layers forming the integuments protect the 

 plants, only against a high transpiration rate, but the writer 

 observed that it is thickest in those species which are resistant to 

 cold. Physiological experiments demonstrated that the time of pe- 

 netration of a certain temperature varies greatty when integuments 

 of different species were used. On an average it took 63,3 minutes 

 for O. castillae, 79,3 for O. Ellisiana, 32,6 minutes for O. spec. Bur- 

 bank Special. 



It was found that the cooling point of all cacti is between — 0,75° 

 and 1,00° C , the freezing point between —20° and —50° C. This 

 suggests that the soluble substances in the cell-sap of cacti are very 

 diffuse. The time, until the tissues are frozen, varies greatly in 

 different species: O. castellae 35 — 44 minutes, O. ficus indica from 

 Malta 12 — 18, O. fusicaulis 45—64 minutes. The author suppcses 

 such is related to respiration, which the plant is able to perform 

 at low temperatures until the water is entirely frozen, at which 

 time the protoplasm probably becomes dormant; each species has 

 its own freezing curve on account of a specific cold resistance of 

 the protoplasm in each species. 



The character of the curve of the time of freezing when pieces 

 of recently killed cactus stems are substituted for living ones, is 

 much shorter (15 to 20 minutes) and is practically the same in all 

 species worked with. The similarity of the curves of freezing in 

 dead tissues of the cacti is due to physical conditions, the great 

 differences in the curves of the living tissues are due to bio- physical 

 conditions. The freezing-curve for each species is quite constant. 

 There appears to be no relation between the length of this curve 

 and the cold-resistance of a species. 



In further experiences the author concludes the cold-resistance 

 is due to the character of the protoplasm, allowance being made 

 for the thickness of the integument when the cold extends over 

 only a Short period. It has been found that O. castillae is killed at 

 — 17° C, O. spec. Burbank Special at —8° C, O. ficus indica from 

 Malta at —6° C, O. Ellisiana at —18° C, O. fusicaulis at —10- C. 



J. C. Th. Uphof. 



Weevers, Th., Die physiologische Bedeutung des Kaliums 



in der Pflanze. (Biochem. Zeitschr. LXXVIII. p. 354—357.1917.) 



Stoklasa behauptet, dass ohne Kalium zwar bei den Bak- 



