Book I. PROCESS OF VEGETABLE DEVELOPEMENT. 241 



vitation. Certain it is that gravitation has considerable influence in preventing the descent of the sap in 

 young shoots of trees which have grown upright; these, when bent down after being fully grown, form 

 larger buds, and often blossom instead of leaf buds. This practice, with a view to the production of blos- 

 som-buds, is frequently adopted by gardeners {Hort. Trans, i. 2,57.) in training fruit trees. These causes 

 are each, perhaps, of some efficacy ; and yet even when taken altogether they are not adequate to the pro- 

 duction of the effect. The greatest stress is laid upon gravitation ; but its agency is obviously over-rated, 

 as is evident from the case of the pendent shoots of the weeping willow ; and if gravitation is so very 

 efficacious in facilitating the descent of the proper juice, how comes its influence to be suspended in the 

 case of the ascending sap ? The action of the silver grain will scarcely be sufficient to overcome it ; and 

 if It should be said that the sap ascends through the tubes of the alburnum by means of the agency of the 

 vital principle, why may not the same vital principle conduct also the proper juice through the returning 

 vessels of the bark ? In short, if, with Saussure, we admit the existence of a contracting power in the 

 former case sufficient to propel the sap from ring to ring, it will be absolutely necessary to admit it also in 

 the latter. Thus we assign a cause adequate to the production of the effect, and avoid at the same time 

 the transgression of that most fundamental principle of all sound philosophy which forbids us to multiply 

 causes without necessity. M. Dutrochet's hypothesis (1550.) for the ascent of the sap accounts equallv for 

 its descent. 



Sect. IV. Process of Vegetable Developement. 



1565. The production of the different parts and organs of plants is effected by the assi- 

 milation of the proper juice. The next object of our enquiry, therefore, will be that of 

 tracing out the order of the developement of the several parts, together with the peculiar 

 mode of operation adopted by the vital principle. But this mode of operation is not 

 exactly the same in herbaceous and annual plants as in woody and perennial plants. In 

 the former, the process of developement comprises as it were but one act of the vital prin- 

 ciple, the parts being all unfolded in immediate succession, and without any perceptible 

 interruption till the plant is complete. In the latter, the process is carried on by gradual 

 and definite stages easily cognisable to the senses, commencing with the approach of 

 spring, and terminating with the approach of winter ; during which, the functions of the 

 vital principle seem to be altogether suspended, till it is aroused again into action by the 

 warmth of the succeeding spring. The illustration of the latter, however, involves also 

 that of the former ; because the growth of the first year exemplifies at the same time the 

 growth of annuals, while the growth of succeeding years exemplifies whatever is peculiar 

 to perennials, 



1566. Elementary organs. If the embryo, on its escape from the seed and conversion 

 into a plant, is taken and miimtely inspected, it will be found to consist of a root, plume- 

 let, and incipient stem, which have been developed in consecutive order ; and if the 

 plant is taken and dissected at this period of its growth, it will be found to be composed 

 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 ? 



1567. No satisfactory explication of this phenomenon has yet been ciffered. It is likely, however, that the 

 rudiments of ail the parts of the plant do already exist in the embryo in such specific order of arrange- 

 ment as shall best fit them for future developement, by the introsusception 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 different, 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 

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

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

 parts enclosed. 



1568. Composite organs. The elucidation of the developement of the composite 

 organs involves the discussion of the two following topics : the foimation 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. 



1569. 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 bud or more. 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. 



1570. 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 ? 



1571. The pith seems only a modification of the original pulp, and the same hypothesis that accounts for 

 the fonnation 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 that the function of the pith was that of generating the stone of fruit, and that a tree deprived of its 

 pith would produce fruit without a stone [Phys. des 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 



u 



