EFFECT OF COLD ON PLANTS. 41 



the tubers of all those plants which stood under trees, and in protected spots, were 

 quite rotten, while no trace of disease appeared in spots which were more elevated 

 and more fully exposed to the current of air. The cause of the disease is the same 

 which, in spring and autumn, excites influenza ; that is, the disease is the effect 

 of the temperature and hygrometric state of the atmosphere, by which, in conse- 

 quence of the disturbance of the normal transpiration, a check is suddenly, or for 

 a considerable time, given to the motion of the fluids, which is one chief condition 

 of life, and which thus becomes insufficient for the purposes of health, or even 

 hurtful to the individual.* 



The whole existence of a plant, the ' esistance which it opposes to the action 

 of the atmospheric oxygen, is most closely connected with the continued support 

 of its vital functions. The mere alternation of day and night makes, in this 

 respect, a great difference. The sinking of the external temperature by a few 

 degrees, causes the leaves to fall in autumn ; and a cold night is followed by the 

 death of many annual plants. 



If we reflect that a plant, in order to protect itself from external causes of 

 disturbance, or to seek the food which it requires, cannot change its place ; that its 

 normal vital functions depend on the simultaneous and combined action of water, 

 of the soil, of the external temperature, and of the hygrometric state of the 

 atmosphere ; that is, on four external circumstances ; it is easy to comprehend the 

 disturbance of functions which must occur in the organism in consequence of any 

 change in the mutual relations of so many combined agencies.f The state of a 

 plant is a sure indication of equilibrium or misproportion in the external conditions 

 of its life ; and the dexterity of the accomplished gardener consists exactly in this* 

 that he knows and can establish the just proportion of- these conditions for each 

 species of vegetable. Only one of these numerous conditions! is in the power of 

 the agriculturist, and that is, the production of the quality of the soil appropriate 

 for the crop, including the necessary modification of its composition, by the 

 mechanical working of the soil ; by the irrigation or draining of the fields ; and 

 lastly, by the employment of manure. When one of the constituents 'of the soil, 

 which, under the given circumstances, is necessary for the support of the vital func- 

 tions; is absent, the external injurious influence is strengthened by this deficiency. 

 Had this constituent been present, the plant would have been enabled to oppose to 

 the external hurtful influences a continued resistance. One day may be decisive 

 as to the life or death of a plant.J An accurate knowledge of the influence exerted 

 by the various constituents of the soil on the diseased condition, must enable the 

 agriculturist to protect and preserve many of his fields for a long time from this 

 destruction ; but it is obvious that a universal remedy against this evil does not exist. 



When the vessels of the plant are filled to overflowing with water, and the motion 

 of the sap is suppressed, the nutrition, in most plants, is arrested, and death takes 

 place. Every one knows the effect of a sudden or of a gradual overfilling of certain 

 parts or organs, when the corresponding evaporation is suppressed. By the endos- 

 motic pressure of the water flowing towards those cells, which contain sugar, 

 mucilage, gum, albumen, and soluble matters in general, the juicy fruits and seeds 

 approaching maturity burst, an-d the juice of grapes, cherries, plums, &c., passes, 

 on contact with the air, into a state of progressive change. The fungi which have 

 been observed on the potato plants and the putrefaction of the tubers, are not the 

 signs of a disease, but the consequences of the death of the plant. 



Among the most important of the experiments made by Hales we must reckon 

 undoubtedly those on the rise of the spring sap in perennial plants. His observa- 

 tions have been entirely confirmed by all those who since his time have studied 

 the subject; but, in my opinion, without our having approached one step nearer to 

 the cause of the phenomena. 



* The cause of potato blight is the same as that of influenza, and depends on the temperature 

 and hygrometric state of the air. 



f The life of plants is dependent chiefly on four external causes : only one of which, namely, 

 the quality of the soil, in the power of the agriculturist. 



J Effects of the presence or absence of a single constituent of the soil. 



j Tlu plant dies, and fungi and putrefaction follow. 



ft 



