540 



THE SOUTHERN PLANTER. 



season) between 10 and 11 degrees, and 

 168 days were required for the perfecting 

 of the plant, giving a total result ol' 1,797 

 degs. In many cases the^numbers came 

 out very satisfactorily for this hypothesis, 

 showing that the plants required a certain 

 sum of heat, and that that heat might be 

 given in small proportions during a large 

 number of days, or in higher proportions 

 in a small number of days. However, 

 there were many important exceptions to 

 this rule, those depending on certain mat- 

 ters of detail which were overlooked in 

 the first experiments. For instance, Bous- 

 singault did not pay attention to the omis- 

 sion of useless heat. Many plants do not 

 move at certain temperatures, even far 

 above the freezing point. No plant, pro- 

 bably, will vegetate or perform any of its 

 vital functions under the freezing point ; 

 many require several degrees above freez- 

 ing point before they begin to start into 

 life. That must be ascertained in each 

 individual plant before we can calculate 

 the useful heat which it receives, as we 

 must omit all degrees below the point at 

 which the plant moves. Then it is impor- 

 tant to take into account the temperature 

 of the soil, upon which the development 

 ,>of plants very greatly depends. We have 

 no more striking illustration of that than 

 was furnished by the records of last year, 

 when the temperature of the soil in this 

 country was found far above the average 

 during the later months, the result of 

 which was the flowering and even the ri- 

 pening of seed in many plants, which very 

 rarely perfect their fruit in this country. 

 Though the consideration of the constitu-^ 

 tion of many plants may be reduced prac- j 

 ticaily to a consideration of the climate — ■ 

 the supply of heat varying with the length | 

 of time during which the plants grow — 

 still there are limits to these conditions. 

 Supposing the calculations to be carried 

 out with the precautions • to which I have 

 just referred, there are certain limits be- 

 yond which they cannot be admissible. — 

 Over-supplies of heat do not produce the 

 same results in a shorter time, but produce 

 disease ; and cold below a certain tempe- 

 ture will prevent the performance of the 

 vital functions at all. It is well known 

 that in the Feroe islands and in Iceland 

 the amount of heat is never sufficient to 

 produce the ripening of the seed of grain, 

 even though the vegetation of the plant 



be prolonged to a much greater extent than 

 in this Climate ; there is not a sufficient 

 stimulus ever given to the plant, so that 

 the natives are obliged to import their 

 seed, though they grow the grain year 

 after year. Again, we may note the infor- 

 mation furnished by travellers in regard 

 to vegetation on the island of Madeira. — 

 There, in the season when the mean tem- 

 perature approaches that of our own sum- 

 mer, our Beech looses its leaves and re- 

 mains without them for 149 days ; the Oak 

 remains without its leaves 110 days, the 

 Vine 157 days. The Vine again affi^rds 

 an example of the noxious results of ex- 

 cessive heat, because in climates near the 

 equator the plant scarcely produces fruit, 

 or only fruit of a very imperfect charac- 

 ter, running away to vegetation, and los- 

 ing those qualities for which it is most 

 valued. The constitution of particular 

 kinds of plants is a subject of great im- 

 portance both to the agriculturist and the 

 horticulturist, and its practical influence 

 has strongly attracted attention in those 

 valuable experim.ents which have been 

 carried on at Rothamsted by Messrs. 

 Lawes and Gilbert. There they have 

 found most important differences, not only 

 between such plants as Turnips and Grain 

 crops, but also differences of constitution 

 between Barley and Wheat — plants be- 

 longing to the same natural families, and 

 differing (scientifically speaking) only in 

 slight peculiarities from each other. 



We cannot doubt the existence of this 

 peculiar constitution in species of plants. 

 But going a step beyond this, we find that 

 individual plants have their constitutions, 

 and the result of this variation within the 

 limits of particular species is to produce 

 plants having characters considerably dif- 

 ferent from each other, and yet referable 

 all to the same specific type. Species of 

 plants are subject to variations according 

 to external conditions, variations which 

 not only affect their structure, but declare 

 themselves in a difference in constitution. 

 Some of these diffeiences of constitution 

 are transmissible, as also the differences 

 in the details of the minor parts of their 

 organization. The preservation of pecu- 

 liarities of this kind, transmitted through 

 generations, results in the formation of 

 what are called races of plants. When 

 we see a variation of species, a particular 

 modification, falling within the specific 



