Effect of Heat , Etc. on Potatoes. 
337 
Like all other phylogenetic criteria, seedling structure needs to 
be used with great caution. If, however, the conclusions drawn 
from it are constantly checked by appeal to the results obtained 
in other lines of research, there seems little reason to doubt that it 
has its value, though this is distinctly greater in some groups than 
in others. 
AGNES ARBER. 
The Effect of Heat and Ether on the Respiration 
and Composition of Potato Tubers. 
In the current number of Flora appears, after an interval of 
25 years, a further contribution by Muller-Thurgau to the study 
of the nature of the resting condition of plants. In his earlier work 
he investigated the respiration of potato tubers, and the composition 
of the reserve substances stored in them, at all stages of their life; 
and he succeeded in constructing from the changes he observed an 
intelligible picture of their internal organisation under normal 
conditions. He conceived a balance to exist between the formation 
of sugars by hydrolysis of starch, and their removal in respiration 
and by storage as starch again. At the beginning of the resting 
period the leucoplasts act so vigorously that no free sugar can 
usually be detected. At 0"C, however, sugar accumulates in 
considerable quantity, proving that at that temperature starch- 
formation is largely inhibited, and suggesting that even at ordinary 
temperatures sugar is still formed, but is completely removed in 
respiration and starch-formation. As the resting season proceeds 
sugar begins to appear even at ordinary temperatures; the leuco¬ 
plasts, becoming less vigorous with age, are no longer able to store 
again all the sugar which is being formed, and an equlibrium 
concentration of sugar is maintained at which the rate of sugar- 
formation (which diminishes with increasing concentration) balances 
the rate of its removal (which increases with the concentration). 
This equilibrium concentration rises as the season advances and the 
vitality of the tuber diminishes. 
The respiration is intimately connected throughout with the 
supply of sugar which is available. It is very high in potatoes 
which have been kept at 0° and have become sweet: at ordinary 
temperatures, it is low in the autumn when little or no sugar is 
present and increases with the accumulation of sugar as the tubers 
grow older. 
In the present contribution the author and a collaborator turn 
their attention to the nature of the effect which etherisation or 
temporary exposure to a relatively high temperature produces, in 
hastening the development of resting organs. Warming has no 
“forcing” effect on potatoes, but their behaviour is investigated as 
a preliminary step to the study of other types in which the forcing 
effect is marked ( e.g ., Convallaria rhizomes). As the work is 
largely a study in the effect of external conditions, of the nature of 
stimuli, on respiration, it comes to hand very opportunely at the 
present time. 
The authors find that after exposure to a temperature of 40"C 
