590 



BOTANY. 



conception of the accretion of vegetable bodies, without 

 a clear understanding of the nature and properties 

 of the membrane of which they are composed, the 

 changes to which this is subject, and the essential 

 qualities with which it is charged. 



Vegetable membrane is not a homogeneous solid, 

 but an aerolated mass, composed of innumerable 

 vesicles or cells slightly attached to each other, and 

 arranged in different forms, as the specific organisa- 

 tion of the plant requires. It is either extended 

 into fibres and filaments, fig. 36, dilated into tissues, 

 depressed into horizontal layers, fig. 37, compressed 

 into perpendicular partitions, or disposed in regular 

 columns. Each cell of these depositions is an insu- 

 lated vesicle, having a thin, pellucid, elastic integu- 

 ment : originally inconceivably minute, but being 

 capable of being distended to a limited size, but in a 

 definite order, incident to the plant to which it belongs, 

 and in any direction ; the cells leaning, and clinging 

 to each other, are consequently pressed into the 

 various figures of spheres, ovals, hexagons, tetragons, 

 cubes, or parallelopipedons. There are also inter- 

 cellular spaces, which serve for conducting of fluids, 

 or depositories of the secretions of the plant. 



When the vegetable membrane is uniform in cha- 

 racter, it is called cellular ; when varied by being 

 disposed into tubes and other organs, it is said to be 

 vascular. The pellicles of the cells are of various 

 consistence and properties as to durability. In the 

 lower orders of vegetation they are mucilaginous and 

 fugitive ; in the higher orders they become ligneous ; 

 and along with the secreted juices of the plant, 

 constitute what is called timber. 



This organic structure of cells, &c., receives the 

 nutriment necessary to the plant, consisting of aque- 

 ous and gaseous qualities extracted from the earth 

 and atmosphere ; and when combined in the aero- 

 lated fabric of the system, become elaborated and 

 assimilated to the essential qualities thereof. By 

 this reception of the elemental food, viz., oxygen, 

 hydrogen, and carbon, the vessels of the system are 

 supplied with these elastic principles ; which when 

 acted on by the heat, light, and humidity of air, 

 intestine motion and consequent distension of the 

 membranes take place, and thus the phenomenon of 

 growth is exhibited. 



It is only by constant attention, and the assistance 

 of the microscope during the development of vege- 

 table membrane, that the above stated facts can be 

 illustrated. When first visible, it resembles a ho- 

 mogeneous jelly ; no signs of organisation can 

 be detected even under the best glasses : but as it 

 acquires consistence, fibres, and ranks of cells gra- 

 dually come into view ; and soon afterwards the 

 perfect structure is apparent. Hence it is reasonable 

 to conclude that the cellular structure has form 

 before any of the parts are visible ; and that it is 

 gradually developed as the season advances, until it 

 arrives at its perfect volume. 



This description is referable to every annual pro- 

 duction, whether it be a single leaf, or shoot, or new 

 layer of wood on the trunk of a tree ; or whether it 

 be a new bulb, tuber, root, or runner under ground. 



The growth of an annual herb is perfected in a 

 few weeks or months ; its life is a gradual expansion, 

 from the time it bursts from the seed till it ripens 

 new seed, which is the last effort of the plant, and 

 after which the whole dies. Some require a period 

 of twenty months to arrive at full bulk and produce 



seed ; such are called biennials. A few continue 

 to grow for several years before they attain their 

 peri'ect form to produce flowers and fruit. But in 

 respect of the majority of shrubs and trees, there 

 really appears no assignable limit to their growth ; 

 as their life only becomes extinct in consequence of 

 being shattered to pieces by storms of wind. 



The life of annual and biennial plants may be 

 perpetuated by art. Sometimes by merely prevent- 

 ing their flowering, or by renewing them by the 

 means of layering, or by rooting cuttings of their 

 branches ; and which layers or cuttings continue to 

 grow for another season, and may, again and again, 

 be preserved for several years by similar means. 

 This expedient, however, is of little use in a practical 

 point of view, as it is only necessary in the case of 

 annuals which do not ripen seed in this country, nor 

 are easily procurable from another. 



Herbaceous plants called perennials, (living from 

 year to year), produce stems and flowers, which die 

 down after the seeds are ripe ; but their roots increase 

 and continue to live, barring accidents, for ever. In 

 the same way the foliage, flowers, and fruit of deci- 

 duous trees are annually produced and shed ; but 

 the shoots and principal parts of the roots by which 

 these were produced remain. The leaves of ever- 

 green trees are shed in the second or some following 

 year ; and even the outer bark of some trees is trien- 

 nially thrown off; while their interior axis of wood 

 is permanent and remains sound for many years, 

 according as it is more or less saturated with resinous 

 sap, or more or less durable in quality. Thus, it is 

 observable that all vegetables are increased by periodi- 

 cal expansions, some of the accretions being perish- 

 able, while others endure ; some truly ephemeral, 

 while others exist for ages. The largest tree is 

 increased by annual additions, each perfectly dis- 

 tinct from all that preceded, and all that may follow. 

 The permanent productions of this year, like all those 

 of former years, remain, as to place and amplitude, 

 ever after unchanged, till they fall to decay. This 

 particularly applies to the new additions of wood, 

 which, when once formed, undergo no further change 

 in bulk or structure. 



The longevity of trees depends upon the durability 

 of the woody deposits. In most, if not all deciduous 

 trees, the first formed layer or zone of wood, namely, 

 that surrounding the pith, is the first to fall to decay. 

 When this begins to take place, the layers, exterior 

 to the first, decay rapidly ; and though the trunk be 

 receiving annually new layers of wood and bark on 

 the outside they, together, do not keep pace with the 

 destruction going on within ; so that at last the trunk 

 becomes a hollow cylinder ; all the first formed layers 

 of both wood and bark being dead, the life is com- 

 pressed into a very narrow space between the albur- 

 num and liber, and where it would ever continue, were 

 the enfeebled trunk not laid prostrate by the tempest. 

 This is the manner of what may be called the natural 

 decay and death of a deciduous tree. It happens 

 sooner or later, according to the solidity or texture 

 of the wood, and to the conservative quality of the 

 concreted sap with which it is saturated ; and also 

 according to the suitableness of the soil on which it is 

 grown. The durability of fir and pine timber (except 

 larch perhaps) depends entirely on the preservative 

 qualities of the resinous sap, and this being usually 

 in greater quantity at the centre than towards the 

 exterior, the heart-wood, as it is called, is most lasting 



