130 Thomson's Lectures on Botany. 



posing tlic substance of the plant ; nor can they result from any cause 

 except from that principle which, whilst it' is present, gives life and 

 motion to every being that is endowed with it ; and, on being withdrawn, 

 leaves the substance to the control of those laws that regulate the 

 combinations of dead, inert, unorganised matter." It was the opinion of 

 Linnaeus, and of many others, that each kind of plant selects its own kind 

 of food, which is adapted to the peculiar nature of its secretions, " But 

 there is no need of this supposition to explain the great variety of secre- 

 tions of plants reared in the same soil, if we admit that vegetables are living 

 beings. ... Is it more wonderful that plants should elaborate poisons and 

 wholesome food, than that one species of reptile should secrete poison, and 

 another living on the same food should not ? In order to prove that no 

 such selection takes place, it is only necessary to rear any number of dif- 

 ferent plants in water alone; each plant in growing will assume the 

 nature and possess the qualities of its species, whatever that may be : a 

 proof that the qualities are not dependent upon the peculiarities of soil, 

 but on the action of the vital principle." 



In all this, every body acquainted with the subject will readily agree ; 

 and it is pleasing to see the lecturer thus escaping from one of the old 

 trammels of hypothetical notions, though sanctioned by the constituted 

 authorities of the science ; for how often have we read of reports from the 

 laboratory, of analysations of every kind of soil, to detect therein the quali- 

 ties of the plants that grew thereon. The search was in vain : they forgot 

 that -in the plant itself there is a fountain for the production of all its 

 qualities, as there is a corculum from whence spring all its forms and 

 functions. 



On this point, however, it is necessary to add, that, in respect to the 

 peculiar food of plants, nothing is more certain in practice than that soils 

 become exhausted of the favourite pabulum, or food, of certain plants, and 

 that a change or restitution is necessary; but this refers to the quantity, 

 and not at all to the quality, of the vegetable. 



Lecture 5. — General Components of the vegetable Structure. All plants 

 are composed of solids and fluids. The fluids differ in qualities, according 

 to the vessels they occupy, and are dissimilar from the fluid as received from 

 the soil. The solids are membranes, cells, vessels, glandulous textures, 

 woody fibre, and the epidermis. Preservative organs are the root, trunk, 

 branches, leaves, and their appendages. Organs of dubious character are 

 bulbs, gems, and gongyli. The fluids are the sap, and proper juice. 



The membrane is that component part of vegetable structure which con- 

 stitutes its basis : it forms, first, the cellular and glandular textures and 

 epidermis ; next, the vascular ; then the ligneous fibre ; in short, enters 

 into the ivhole solid material. In the descriptions of those parts, the lecturer 

 collects the opinions of all the most eminent physiologists, and shows (by 

 the assistance of marginal cuts), with great accuracy, the various structure 

 of plants. In describing the vascular part, he proves, by experiments, that 

 the different fluids occupy different vessels ; which circumstance leads to 

 the conclusion, that the sap, or watery fluid received from the soil, is car- 

 ried in one set of vessels, and that the proper juices are conveyed in 

 another. 



The experiments which prove this are curious, and deserve the attention 

 of every young botanist; and the vascular fabric, as described by the lec- 

 turer, and exhibited by his highly magnified representations, present to the 

 eye some of the most wonderful conformations of matter which can be 

 met with in the whole range of nature's works. 



The ligneous fibre is a very minute, firm, elastic, semi-opaque filament, 

 which, by cohesion with other filaments of the same kind, forms the proper 

 fibres that constitute the grain, or solid part of the wood. It enters also 

 into the composition of another set of layers, that traverse the longitudinal, 



