CHAMBERS'S INFORMATION FOR THE PEOPLE. 



which roots in the parched sand is furnished with 

 leaf-organs to absorb moisture from the atmos- 

 phere, and retain it ; while in a wet situation these 

 organs would become diseased, and rot away ; so, 

 in like manner, a marsh-plant, whose spongioles 

 are its main organs of sustenance, would perish 

 were it removed to an arid soil. The organic 

 structure of such plants forms a limit to their 

 distribution ; and the same may be said of the 

 Salicornia, Salsola, Lepigonum vian'nuin, &c., 

 which live only when exposed to the salt spray of 

 the ocean. 



Heat and light are perhaps the most manifest 

 agents in the distribution of vegetable life. The 

 luxurious growth of the tropical jungle is the 

 direct result of warmth and moisture, just as the 

 barrenness of Nova Zembla is the effect of piercing 

 cold ; yet both situations are inhabited by plants 

 which enjoy the conditions peculiar to their exist- 

 ence. No conditions of mere soil, or light, or 

 moisture, could make the palms, tree-ferns, and 

 jungle-flowers of India flourish in Great Britain ; 

 so neither would our oaks or pines flourish in 

 Iceland, unless we could provide for them that 

 temperature and seasonal influence necessary to 

 their healthy existence. Light, though it acts 

 most powerfully on the colours and blossoms of 

 plants, is in some measure an element in their 

 geographical arrangement. The northern side of 

 a hill may sometimes be as green, but it never 

 will be so flowery as the southern. The more free 

 the exposure, the more readily will most plants 

 also blossom, and yield a rich fruit. So well is 

 this understood in the grape-countries on the 

 Rhine, that the right bank of that river, which 

 faces the sun, is reckoned to be much more 

 valuable than the left, and commands a higher 

 price for its wines. 



Altitude, or elevation above the ordinary sea- 

 level, also exerts an obvious influence on the dis- 

 tribution of vegetable life : it is equivalent to 

 removal from a tropical to a temperate region, 

 or from temperate latitudes to the arctic circle. 

 For every hundred feet of ascent, there is a pro- 

 portional fall of the thermometer ; so that, at the 

 height of 5000 feet in the latitude of Britain, and 

 16,000 at the equator, we arrive at the region of 

 perpetual snow. This intimate relation between 

 altitude and decrease of temperature accounts for 

 the fact, why the base of a mountain may be 

 clothed with the vegetation of tropical India, the 

 sides with that of temperate England, and the 

 summit with the mosses and lichens of icy Lab- 

 rador. 'We may begin the ascent of the Alps, 

 for instance, in the midst of warm vineyards, and 

 pass through a succession of oaks, sweet chestnuts, 

 and beeches, till we gain the elevation of the more 

 hardy pines and stunted birches, and tread on 

 pastures fringed by borders of perpetual snow. 

 At the elevation of 1950 feet, the vine disappears ; 

 and at 1000 feet higher, the sweet chestnuts cease 

 to thrive ; looo feet farther, and the oak is unable 

 to maintain itself ; the birch ceases to grow at an 

 elevation of 4680 ; and the spruce-fir at the height 

 of 5900 feet, beyond which no tree appears. The 

 Rhododendron ferrugineum then covers immense 

 tracts to the height of 7800 feet ; and the herba- 

 ceous willow creeps 200 or 300 feet higher, 

 accompanied by a few saxifrages, gentians, and 

 grasses ; while mosses and lichens struggle up 

 to the imperishable barrier of eternal snow.' 



68 



The circumstances which facilitate the dis- 

 persion or migration of plants, are not uncon- 

 nected with the causes which limit their geo- 

 graphical distribution. Many seeds drop from 

 the parent stalk, spring up into new series of 

 stems, which in turn give birth to another race 

 of seeds, and these again to another circle of 

 vegetation. Thus, any species of plant would 

 spread from a common centre till arrested by the 

 influences which limit its range of habitat ; and 

 this mode of dispersion no doubt frequently 

 occurs. In most plants, however, the seeds are 

 small and light, and easily borne about by the 

 winds : some are downy, and furnished with wings 

 or hairs, while others are ejected from their car- 

 pels with considerable force. All these appendages 

 and peculiarities are evidently intended to facilitate 

 their dispersion, which is further assisted by rivers, 

 lakes, and tidal currents, by the wool of animals, 

 the droppings of birds, and the economical pur- 

 suits of man, whether accidental or intentional. 



STRUCTURE OF PLANTS. 



In order to explain the varied phenomena of 

 plant-life, we must revert to the elementary tissues 

 of which the various organs are composed. It 

 is in these tissues that the processes of growth 

 and multiplication, of individual development and 

 of reproduction, can alone be accurately traced. 

 Vegetable Histology limits itself to such investiga- 

 tions. If we cut a thin slice from the leaf or 

 succulent stem of a plant, and place it under the 

 object-glass of a microscope, we shall find it to 

 present the general appearance of a web or tissue, 

 more or less of a honeycomb form, 

 as represented in the wood-cut (a). 

 If more carefully examined, it will 

 be found that the apparent meshes 

 of the network are not hollow 

 spaces in a homogeneous mass or 

 membrane, but that each has its 

 own envelope is, in fact, a minute 

 bladder or vesicle, or elongated 

 tube, as the case may be ; and that 



the whole mass of tissue is formed /- n i * 



, ., ,- , a. Cellular tissue. 



by the aggregation of such vesi- ' 



cular or tubular bodies held together by means 

 of an imperceptible layer of intercellular matter, 

 which causes the contiguous cell-membranes to 

 cohere. In the tissues of plants there is an infinite 

 variety, both as regards the form and arrangement 

 of their elementary bodies, but all may be reduced 

 to three distinct types of structure, to one or other 

 of which every modification can be referred. These 

 are distinguished as the cellular, woody, and 

 vascular tissues. 



Cellular tissue (Parenchyma) may be said, in 

 general terms, to form the soft and succulent parts 

 of plants as, for example, the pith of trees, the 

 delicate central tissue of Papyrus, shewn in the 

 following figure (b\ and the soft parts of our 

 esculent vegetables and fruits. It is composed of 

 minute bladders or vesicles, which are called cells y 

 which have their walls composed of a chemical 

 substance called cellulose. Cells are seldom so 

 large as to be visible to the naked eye, but are 

 readily seen under a compound microscope. They 

 usually consist of a pellucid membrane, and con- 

 tain a formative protoplasm in their interior, with 

 nuclei and nucleoli, which give origin to new cells. 



