VEGETABLE PHYSIOLOGY. 



baobab, like all soft-wooded trees, grows rapidly, 

 and attains an enormous diameter in less than a 

 hundred years. The mode of counting by con- 

 centric circles only applies to exogenous trees, and 

 even with them is very uncertain. A warm spring, 

 which sets the sap early in motion, followed by 

 weather cold enough to check vegetation, will give 

 the appearance of two layers in one year, as the 

 recommencement of vegetation will have the same 

 appearance as a new layer in spring. In many 

 trees, such as the oak, for example, a second 

 growth often takes place after midsummer ; so 

 that even a third layer is occasionally formed in 

 the course of six months. On the other hand, it 

 is possible that a moist warm winter, by keeping 

 an evergreen tree growing the whole year without 

 any check to vegeta- 

 tion, might give the 

 appearance of only one 

 layer to the growth of 

 two years. Notwith- 

 standing these anom- 

 alies, practical men find 

 counting the concentric 

 circles of a tree the 

 best mode which has 

 yet been discovered of 

 ascertaining its age, as 

 in northern countries only one growth is made 

 in the course of a year. The accompanying figure 

 represents a section of an exogenous or outside- 

 growing stem five years old, having the pith in 

 the centre, a cylindrical layer for every year of the 

 growth, the bark on the outside, and the medullary 

 rays passing from the centre to the circumference. 



DISTRIBUTION OF PLANTS. 



The geographical arrangement of the vegetable 

 world is influenced by conditions of soil, heat, 

 moisture, light, altitude of situation, and various 

 other causes ; for, did they flourish independently 

 of these conditions, then there were no reason why 

 the vegetation of one part of the globe should differ 

 from that of another. The flowers, shrubs, and trees 

 which adorn the plains of India, are not the same 

 as those which clothe the valleys of Britain ; and 

 these, again, are totally different from the scanty 

 vegetation of Iceland or Spitzbergen. Each species 

 is, nevertheless, perfectly adapted to the conditions 

 under which it exists, and finds in its native situa- 

 tion all the elements required for its growth. 



In a state of domestication, however, many 

 species exist in regions far beyond the limits of 

 their original distribution. Our cultivated useful 

 plants are of this kind ; as, for example, the potato, 

 which is a native of tropical America, and is of 

 the highest utility in Northern Europe. In South 

 America, the warm climate enables it to propagate 

 by the seed ; hence in that region its tubers are 

 small and insignificant ; but in Europe, where the 

 climate is unfavourable to the production of the 

 plant from seed, it propagates by the tubers, which 

 are consequently enlarged, so as to contain a store 

 of nutriment for the young plant, before its stem 

 and leaves be sufficiently developed. 



The habitats of plants that is, the situations in 

 which they naturally thrive best are generally 

 distinguished as follows : Marine, when the plants 

 float upon, or are immersed in, salt water, such 

 as sea-weeds ; and maritime, when they grow by 



the sea-shore, or in places exposed to the influence 

 of the sea-breeze. Aquatic is the general term 

 for fresh-water habitats ; and these may be lacus- 

 trine, that is, growing in lakes-; fluviatile, in 

 rivers or palustrine, when in marshes or wet 

 meadow-lands. Plants are also distinguished as 

 growing in open pastures, in cultivated lands, 

 woods, mountainous parts, and in caves, mines, 

 and other underground excavations. The term 

 epiphyte indicates that the species grows upon 

 others without deriving from them the elements 

 of nutrition; and parasite, that it adheres to their 

 surface, enters their tissue, and directly extracts 

 its nourishment. Examples of epiphytes are seen 

 in numerous species of tropical orchids, and para- 

 sites in such plants as the mistleto, dodder, and 

 Broom-rapes. The range of habitat is that extent 

 of the earth's surface over which a plant is dis- 

 tributed by nature. The terms maritime and 

 alpine, for example, are general in their applica- 

 tion, and refer to all plants which grow by the 

 sea-side or on mountains ; but the plants which 

 flourish on the sea-shores of Great Britain are not 

 the same with those on the coast of Africa ; nor 

 are these, again, identical with the maritime vege- 

 tation of Chili. The geographical range of aay 

 plant conveys a more special idea, and embraces 

 only that particular tract over which the species 

 extends. This range is circumscribed by con- 

 ditions of temperature, light, and elevation above 

 the sea, and does not, as might be supposed, 

 depend very closely upon belts of longitude, by 

 which temperature is generally indicated. Thus, 

 nearly all the beautiful Pelargoniums and Mesem- 

 bryanthemums which adorn our green-houses are 

 natives of a limited space near the Cape of Good 

 Hope, as are also many of our most beautiful 

 bulbs. The curious Stapelias, that smell so much 

 like carrion, are found wild only in South Africa. 

 The different kinds of Eucalyptus and Epacris 

 are restricted to Australia. The Umbelliferous 

 and Cruciferous plants spread across Europe and 

 Asia ; the Cacti are found in tropical America ; 

 and the Labiatae and Caryophyllaceae are seldom 

 found beyond Europe. The peculiar ranges and 

 centres of vegetation, as they are termed, cannot 

 be sufficiently understood, however, without a 

 knowledge of the different tribes and classes of 

 plants, the consideration of which forms the sub- 

 ject of next paper SYSTEMATIC BOTANY. 



Soil exercises less influence on the distribution 

 of plants than is usually ascribed to it, though 

 there can be no doubt that on its power of absorb- 

 ing and retaining heat and moisture much of the 

 luxurious growth of vegetables depends. They 

 will grow to some degree in almost any soil, as the 

 bulkier ingredients clay, lime, and sand always 

 predominate ; but a proper proportion of these 

 earths is necessary to perfect vegetation, and many 

 plants will not continue healthy and propagate 

 unless supplied with other elements, such as potash, 

 soda, and various metallic salts. For this reason, 

 the natural vegetation of a limestone country differs 

 from that of a retentive clay ; while the plants 

 which cover sandy downs are different from those 

 of the alluvial valley. Moisture, which is indis- 

 pensable to the existence of vegetation, also exer- 

 cises great influence in its natural distribution. 

 Very dry regions are deficient in vegetable forms, 

 while their luxuriance in tropical countries is con- 

 nected with great heat and moisture. The plant 



