Book I. PROCESS OF VEGETABLE DEVELOPEMENT. 241 



merely of an epidermis enveloping a soft and pulpy substance, that forms the mass of 

 the individual ; or it may be furnished also with a central and longitudinal fibre ; or with 

 bundles of longitudinal fibres giving tenacity to the whole. These parts have been de- 

 veloped no doubt by means of the agency of the vital principle operating on the proper 

 juice ; but what have been the several steps of operation ? 



1539. No satis/acton/ explication of this phenomenon has yet been offered. It is likely, however, that the 

 rudiments of all the different parts of the plant do already exist in the embryo in such specific order of 

 arrangement as shall best fit them for future developement, by the intro-susception of new and additional 

 particles. The pellicle constituting the vegetable epidermis has generally been regarded as a membrane 

 essentially distinct from the parts which it covers, and as generated with a view to the discharge of some 

 particular function. Some phytologists, however, have viewed it in a light altogether difi'erent, and have 

 regarded it as being merely the effect of accident, and nothing more than a scurf formed on the exterior 

 and pulpy surface of the parenchyma indurated by the action of the air. It is more probably, however, 

 formed by the agency of the vital principle, even while the plant is yet in embryo, for the very purpose of 

 protecting it from injury when it shall have been exposed to the air in the process of vegetation. There 

 are several respects in which an analogy between the animal and vegetable epidermis is sufficiently 

 striking : they are both capable of great expansion in the growth of the subject ; they are both easily re- 

 generated when injured (excepting in the case of induration), and seemingly in the same manner; they 

 are both subject, in certain cases, to a constant decay and repair j and they both protect from injury the 

 parts enclosed. 



1540. Composite organs. The elucidation of the developement of the composite 

 organs involves the discussion of the two following topics : the formation of the annual 

 plant, and of the original shoot of the perennial ; and the formation of the subsequent 

 layers that are annually added to the perennial. 



1541. Annuals and annual shoots. If a perennial of a year's growth is taken up in 

 the beginning of winter when the leaves, which are only temporary organs, have fallen, 

 it will be found to consist of a root and trunk, surmounted by one or more buds. The 

 root is the radicle expanded into the form peculiar to the species, but the trunk and buds 

 have been generated in the process of vegetation. 



1542. The root or trunk, if taken and cut into two by means of a transverse section, will be found 

 to consist already of bark, wood, and pith. Here then is the termination of the growth of the annual, 

 and of the first stage of the growth of the perennial : how have their several parts or organs been 

 formed ? 



1543. The pith seems only a modification of the original pulp, and the same hypothesis that accounts for 

 the formation of the one will account also for the formation of the other ; but the pith and pulp, or 

 parenchyma, are ultimately converted into organs essentially distinct from one another; though 

 phytologists have been much puzzled to assign to each its respective functions. In the ages in which 

 phytological opinions were formed without enquiry, one of the vulgar errors of the time seems to have 

 been an opinion by which the function of the pith was supposed to be that of generating the stone of fruit, 

 and by which it was thought that a tree deprived of its pith would produce fruit without a stone. [Phys, 

 lies Arb. liv. i. chap. 3.) But this opinion is by much too absurd to merit a serious refutation. Another 

 early opinion, exhibiting however indications of legitimate enquiry, is that by which the pith was 

 regarded as being analogous to the heart and brain of animals, as related by Malpighi ; who did not 

 himself adopt it, but believed the pith to be like the cellular tissue, the viscera in which the sap is 

 elaborated for the nourishment of the plant, and for the protrusion of future buds. Magnol thought that 

 it produces the fiower and fruit, but not the wood. Du Hamel regarded it as being merely an extension 

 of the pulp or cellular tissue, without being destined to perform any important function in the process of 

 vegetation. But Linnaeus was of opinion that it produces even the wood ; regarding it not only as the 

 source of vegetable nourishment, but as being also to the vegetable what the brain and spinal marrow- 

 are to animals, the source and seat of life. In these opinions there may be something of truth, but 

 they have all the common fault of ascribing to the pith either too little or too much. M. Lindsay of 

 Jamaica suggested a new opinion on the subject, regarding it as being the seat of the irritability of the 

 leaves of the mirao-sa, and Sir J. H Smith says he can see nothing to invalidate the arguments on which 

 this opinion is founded. Plenck and Knight regard it as destined by nature to be a reservoir of moisture to 

 supply the leaves when exhausted by excess of perspiration. Hence it appears that the peculiar function 

 of the pith has not yet been altogether satisfactorily ascertained ; and the difficulty of ascertaining it has 

 been thought to be increased from the circumstance of its seeming to be only of a temporary use in the 

 process of vegetation, by its disappearing altogether in the aged trunk. But although it is thus only 

 temporary as relative to the body of the trunk, yet it is by no means temporary as relative to the process 

 of vegetation ; the central part of the aged trunk being now no longer in a vegetating state, and the 

 pith being always present in one shape or other in the annual plant, or in the new additions that are 

 annually made to perennials. The pith then is essential to vegetation in all its stages : and from the 

 analogy of its structure to that of the pulp or parenchyma, which is known to be an organ of elabor- 

 ation, as in the leaf, the function of the pith is most probably that of giving some peculiar elaboration to 

 the sap. 



1544. The generation of the layer of wood in woody plants, or of the parts analogous to wood in the case 

 of herbaceous plants, has been hitherto but little attended to. If we suppose the rudiments of all the 

 different parts to exist already in the embryo, then we have only to account for their developement by 

 means of the intro-susception and assimilation of sap and proper juice ; but if we suppose them to be 

 generated in the course of vegetation, then the difficulty of the case is augmented : and at the best we 

 can only state the result of operations that have been so long continued as to present an effect cognizable 

 to the sense of sight, though the detail of the process is often so very minute as to escape even the nicest 

 observation. All, then, tliat can be said on the subject, is merely that the tubes, however formed, do, by 

 virtue of the agency of the vital principle operating on the proper juice, always make their appearance flt 

 last in a uniform and determinate manner, according to the tribe or species to which the plant belongs, 

 uniting and coalescing so as to form either a circular layer investing the pith, as in woody plants; or a 

 number of divergent layers intersecting the pith, as in some herbaceous plants ; or bundles of longitudinal 

 and woody fibre interspersed throughout the pith, as in others. In the same manner we may account for 

 the formation of the layer of bark. ^ 



1545. Perennials and their annual layers. If a perennial is taken at the end of the 

 second year and dissected as in the example of the first year, it will be found to liave 

 increased in height by the addition of a perpendicular shoot consisting of bark, wood, 

 and pith, as in the shoot of the former year ; and in diameter by the addition of a new 



II 



