Thomsoti^s Lectures on Botany. 131 



named divergent. Whether the ligneous fibre is of original formation, or 

 condensed, membranous, or cellular texture, or obsolete obstructed vessels, 

 is yet undetermined. 



The epidermis is that portion of the vegetable structure which is ex- 

 terior to all the others, being a fine, transparent, unorganised pellicle, 

 generally colourless, sometimes annually deciduous, dispersed as a powder, 

 or existent and fractured into rugged bark, and is a distinct organ. 



Such are the principal solid components of the vegetable body. Perhaps 

 they may be resolved into modifications only of the membranous and cel- 

 lular textures ; " but though we allow that the vessels, ligneous fibre, &c., 

 are composed of such substance, we prefer regarding them as distinct com- 

 ponents." (p. 102.) 



Fluids. When the plant is in a growing state, if an incision is made in 

 the bark, common sap exudes from the wound. " If it were possible to 

 obtain this from the vessels very near to the extremities of the roots, we 

 should be able to discover the real food of plants. This, however, cannot 

 be accomplished," because it soon becomes mixed with the ready-formed 

 vegetable matter, and so becomes altered from its original state. On this 

 account, although there is reason for believing that the food of almost all 

 vegetables is the same, yet no just idea can be formed of its nature from 

 the most accurate analysis of it, when procured from any single plant. The 

 lecturer proceeds with an account of various trials to analyse the sap, and 

 rightly concludes, that the different qualities found in plants are concocted 

 by the system itself, rather than by any extraneous matters received by the 

 roots ; adding, that in the present state of our knowledge, all that can be 

 aimed at is the formation of a probable hypothesis, rather than the attain- 

 ment of truth, deduced from certain experiments. In this mode of view- 

 ing the subject, we may regard the sap as consisting of water, which is its 

 principal component, carbonaceous matter, acetate of potash, and carbonate 

 of lime ; which ingredients are decomposed by the vital powers of plants. 



The proper Juice. " When a plant is cut through transversely, the 

 proper juice is seen issuing from both divided surfaces, but in greatest 

 quantity from the open orifices of the divided vessels in the part farthest 

 from the root ; a fact which is ascribable to the progression of the proper 

 juice being invertedly to that of the sap," and is " that changed state of 

 the sap, after it has been exposed to the air and light, in the leaf, and is 

 returning from it to form the different secretions." This proper juice has 

 been found to contain various qualities, and among these ligneous fibre has 

 been detected by the chemist Chaptal. This has also been found in the 

 seed lobes in greater quantity than in the juice itself. No woody fibre 

 is found in the ascending sap, although the pi'inciples of it are undoubtedly 

 contained in that fluid. " A new chemical combination of these principles 

 takes place; but how this is effected, or by what means the change is pro- 

 duced, we know not ; and it is one of those mysteries of nature from which 

 human ingenuity will never be able to remove the veil." 



The secretions of plants, formed from the proper juice, are numerous, and 

 known under the names of gum, fecula, sugar, gluten, albumen, gelatin, 

 caoutchouc, wax, fixed oil, volatile oil, camphor, resin, gum resin, balsam, 

 extract, acids, tannin, aroma, the bitter, the acrid, the narcotic principles, 

 and ligneous fibre. These are found in different parts of plants, without 

 regularity of distribution, yet are all composed of the same principles, viz. 

 carbon, hydrogen, and oxygen. The lecture, concludes with examples and 

 results of chemical analysations of the proper juice and solids of different 

 plants. 



The seven lectures which follow contain a minute and accurate detail of 

 the physiology of plants, their structure beautifully delineated by well 

 executed plates, and the characters and functions of the different parts 

 ably described. The structure of leaves the lecturer has closely studied, 



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