OF BOTANY. XX ill 



212. In Endogenous plants (80) it is probable that the upward and horizontal 

 course of the fluids is through the cellular tissue, and that the downward passage 

 takes place through the bundles of vascular tissue. 



213. The precise direction of the sap in cellular plants (80) is unknown. 



214. Besides mucilage, water, and sugar, plants contain several other principles 

 either proximate or accessory. 



215. The proximate principles are formed by the vital powers of the plant acting, 

 in conjunction with air and light, upon the fluids introduced into its system. 



216. Many accessory or foreign principles are also found in plants, such as silcx, 

 phosphate of lime, phosphorus, &c. 



217. As it has been ascertained, by experiment, that these are formed in plants 

 the aliment of which did not contain them, it is inferred that the presence of such 

 principles also depends upon the operation of the vital powers of vegetation. 



218. The most important chemical phenomenon connected with the growth of 

 plants, is the property possessed by their leaves, or green parts, of absorbing oxy- 

 gen and parting with carbonic acid gas in the dark ; and of parting with their oxy- 

 gen under the influence of the sun. 



219. The alternate action of this phenomenon is supposed to cause, in conjunc- 

 tion with the peculiar vital powers of particular species, all the variety of proximate 

 and foreign principles found in vegetables. 



220. No plants can long exist in which an alternate absorption and expulsion of 

 oxygen does not take place, except Fungi. 



221. The expulsion of oxygen is determined by the quantity of light to which a 

 plant is exposed. Light causes the decomposition of the carbonic acid gas, and the 

 accumulation of solid matter. 



222. Hence, if a plant is exposed to too strong a light, it perishes, from the ex- 

 cessive expulsion of oxygen. 



223. And if it is not exposed to the influence of light, it dies from the accumula- 

 tion of that principle. 



224. If there is too great an accumulation of oxygen, an attempt will always be 

 made by a plant to reach the light, for the purpose of parting with the superfluity ; 

 as in seeds, which, in germination, shoot from darkness into light. 



225. If this cannot be effected, etiolation first takes place, which is caused by 

 the accumulation of oxygen, and the consequent non-deposition of carbon; and 

 death succeeds. 



226. Seeds will not germinate in the light, because light decomposes their car- 

 bonic acid gas, expels the oxygen, and fixes the carbon, whence all the parts become 

 hardened. 



IX. FLOWER-BUDS. 



227. The FihOwer-bud (143) consists of imbricated, rudimentary, or metamor- 

 phosed leaves, the external or inferior of which are usually alternate, and the inter- 

 nal or superior always verticillate, or opposite ; the latter are called^/foraZ envelopes 

 and reproductive organs. 



228. As every flower-bud proceeds from the axilla of a leaf, either fully developed 

 or rudimentary, it therefore occupies exactly the same position with respect to the 

 leaf as a leaf-bud. 



229. The leaf from the axilla of which a flower-bud arises, is called a bractea, 

 or floral leaf; and all rudimentary leaves, of what size or colour soever, which ap- 

 pear on the peduncle between the floral leaf and the calyx, are called bracteolce. 



230. But in common language, botanists constantly confound these two kinds, 

 which are, nevertheless, essentially distinct. 



231. Although the buds in the axilla of bracteee are often not developed, yet they 

 have the same power of developement as those in the axillae of leaves ; they are 

 generally flower-buds, very rarely leaf-buds. 



232. When a single bractea is rolled together, highly developed, and coloured, 

 and is placed at the base of the form of inflorescence called a spadix (259,) it is 

 named spatha; Ex. Arum. 



233. When several bracteee are verticillate or densely imbricated around the 

 base of the forms of inflorescence called the umbel, or capituluin (261,) they receive 

 the name of involucrum; Ex. Carrot, Daisy. 



234. When the bracteee of an involucrum form a single whorl, and cohere by 

 their margins, it is impossible to distinguish them from the calyx by any other 



