562 ESTIMATION OF THE HEAT NECESSARY TO GRO^VTH. 



are necessary. Isolated species of course exhibit puzzling deviations in this respect. 

 The Acacia {Robinia Pseudacacia) develops its flowers in Southern Italy before its 

 leaves, and when the acacia trees are already in full bloom their foliage-leaves are 

 still minute and unexpanded. North of the Alps the foliage-leaves everywhere 

 unfold at the same time as the flowers; and yet we always reckon the heat 

 indicated by the thermometer as if it were utilized in an identical manner by 

 associated plants in all stages of development. 



Finally, it must be pointed out that certain alterations which are carried on in 

 the interior of a seed or plant during its apparent rest, and which have a great 

 significance for those later phenomena of growth visible to the eye, are completely 

 excluded from observation and registration. If potato-tubers are dug up in autumn 

 and put in a cellar, it seems as if all movements and chemical transformations were 

 entirely stopped in their individual cells. The potato-tuber lies tranquilly resting 

 in the dark cellar, in which a constant temperature of 10° prevails, throughout the 

 winter. Spring arrives; above-ground everything germinates and sprouts from the 

 sun-warmed soil, and we connect this phenomenon with the powerful heating caused 

 by the rays of a more vertical sun. No heat-giving sunbeams reach the cellar, 

 however. The temperature of the air, of the earth, and of the potatoes which have 

 been lying there for months is always the same, 10° — even perhaps now a fraction 

 lower, since according to experience the lowest temperature in the cellar is not 

 reached until the end of the winter. Nevertheless the potato begins to grow and to 

 send out a slender shoot from one of its buds, as if it knew that spring, the proper 

 time for sprouting and growing, had arrived. Why does the growth not begin 

 until now in March? Why did it not commence in December, since external 

 influences, particulai'ly the temperature of the environment, was not in any way 

 different within the cellar then from what it is now in spring? There can be only 

 one answer to the question, which is, that in December the potatoes were not yet 

 equipped for growth. They were only apparently in absolute rest; in reality 

 chemical transformations, the preparation and production of constructive materials, 

 were being carried on in the cells, and in December, January, and February, these 

 were not far enough advanced for the tubers to be able to produce stems, leaves, 

 and roots. Not until now in March are the preparations for development completed, 

 and not until now can that transformation of the constructive materials occur 

 which is outwardly manifest as growth. The organic compounds, contained by the 

 cells of the tubers in autumn, were not fit for the formation of stems, leaves, and 

 roots, even under the influence of a temperature of 20°. All these processes require, 

 therefore, a definite period of time, and this can neither be replaced nor sensibly 

 shortened by a rise of temperature. 



In the underground bulb of the Snowdrop (Galanthus nivalis) the rudiments 

 of the leaves and blossoms of the following spring are already formed during 

 the summer, and at the end of September all portions of the futui-e flowers 

 can be recognized between the enveloping sheaths and bulb-scales. It might 

 be thought an easy matter to force this bulb by raising the temperature and 



