202 



SCIENCE OF GARDENING. 



Pam II. 



penetrated by their own vegetating power. But the fact Is, that they are merely covered by the addi- 

 tional layers of wood that have been formed since the fibres first insinuated themselves into the bark. 



942. The Cuscuta europcea, or Dodder (Jig. 68.), though it 

 is to be accounted a truly parasitical plant in the issue, is 

 yet not originally so. For the seed of this plant when it 

 has fallen to the ground takes root originally by sending 

 down its radicle into the soil and elevating its stem into 

 the air. It is not yet, therefore, a parasitical plant But 

 the stem which is now elevated above the surface lays 

 hold of the first plant it meets with, though it is par- 

 ticularly partial to hops and nettles, and twines itself 

 around it, attaching itself by means of little parasitical 

 roots at the points of contact, and finally detaching itself 

 from the soil altogether by the decay of the original root, 

 and becoming a truly parasitical plant. Withering de- 

 scribes the plant in his arrangement as being uiiginally 

 parasitical ; but this is certainly not the fact. 



943. The Orobanche, or Broom-rape, which attaches 

 itself by the root to the roots of other plants, is also to be 

 regarded as being truly parasitical, though it sometimes 

 sends out fibres which seem to draw nourishment from 

 the earth. It is found most frequently on the roots of 

 common Broom. 



944. The various tribes of Epidendrums are regarded also 

 by botanists as parasitical plants, because they are ge- 

 nerally found growing on other trees. But as they are 

 found to grow in old tan, they probably derive only sup- 

 port from the bark of trees, and not nourishment. 



945. Light is a body which has very considerable influence on the structure of vege- 

 tables, and some also on their habitation. The fungi do not require the usual interludes 

 of day, in order to decompose carbonic acid gas, and can live and thrive with little or no 

 light. In green plants, which require the action of light, the intensity required is very 

 different in different species ; some require shady places, and hence the vegetable in- 

 habitants of caves, and the plants which grow in the shade of forests ; others, and the 

 greater number, require the direct action of the sun, and grow in exposed elevated sites. 

 De Candolle considers that the great difficulty of cultivating Alpine plants in the gar- 

 dens of plains, arises from the impossibility of giving them at once the fresh temperature 

 and intense light which they find on high mountains. 



Sect. III. Civil Causes affecting the Distribution of Plants. 



946. By the art of man plants may be inured to circumstances foreign to their usual 

 habits. Though plants in general are limited to certain habitations destined for them by 

 nature, yet some are, and probably the greater number may be, inured to climates, soils, 

 and situations, of which they are not indigenous. The means used are acclimating and 

 culture. 



947. Acclimating seems to be most easily effected in going from a hot to a cold climate, 

 particularly with herbaceous plants. Because it often happens that the frosts of winter are 

 accompanied with snow, which shelters the plant from the inclemency of the atmosphere 

 till the return of spring. Trees and shrubs, on the contrary, are acclimated with more 

 difficulty, because they cannot be so easily sheltered from the colds, owing to the greater 

 length of their stems and branches. The acclimating or naturalisation of vegetables is to 

 be attempted by two modes : by sowing the seeds of successive generations, and by the 

 difference of temperature produced by different aspects. The former is well exemplified 

 in the case of the rice-plant which is grown in Germany, from seeds raised there, while 

 if seeds from its native country, India, are used they will not vegetate (Sir J. Banks, 

 in Hort. Trans, vol. i.) ; and the latter in the sloping banks of Professor Thouin of 

 Paris, as described by Girardin. (Physiologie Vegetate, vol. i.) Some plants seem to 

 have the capacity of vegetating in almost all climates, or of naturalising themselves in 

 almost any. This is particularly the case with esculents, such as the domestic cabbages, 

 potatoes, and carrots. (Dialogues on Botany, p. 411.) 



948. Domesticated plants. " Some plants," Humboldt observes, " which constitute 

 the object of gardening and of agriculture, have time out of mind accompanied man 

 from one end of the globe to the other. In Europe, the vine followed the Greeks ; the 

 wheat, the Romans ; and the cotton, the Arabs. In America the Tultiques carried 

 with them the maize ; the potatoe and the quinoa (Chenopodium quinoa, of which the 

 seeds are used,) are found wherever have migrated the ancient Condinamarea. The 

 migration of these plants is evident ; but their first country is as little known as that of 

 the different races of men, which have been found in all parts of the globe from the ear- 

 liest traditions." (Geographic des Plantes, p. 25.) 



949. The general effect of culture on plants is that of enlarging all their parts ; but it 

 often also alters their qualities, forms, and colors : it never, however, alters their pri- 

 mitive structure. " The potatoe," as Humboldt observes, " cultivated in Chili, at 

 nearly twelve thousand feet above the level of the sea, carries the same flower as in 

 Siberia." 



