LECTURES ON BOTANY. 



403 



LECTURES ON BOTANY* 



COURSE OF LECTURES ON BOTANY IN REFERENCE TO AGRICULTURE. 



By Charles Johnson, Esq., Professor of Botany at Guy^s Hospital, ^-c. ^c. At Messrs. 

 Nesbits' Agricultural and Scieidijic Training School, Kennington Lane, Lambeth, near 

 London. 



Lecture IV. 



It has been already remarked that the rela- 

 tive distribution of the minute cells and tubes 

 varies gi-eally in different plants, and occa- 

 sions organic differences of vast importance 

 to the investigator of their fonns and proper- 

 ties. A very lai'ge proportion of the Vegeta- 

 ble Kingdom, numerically considered, con- 

 sists exclusively of the cell, either simple 

 and separating into as many distinct individu- 

 als as there are cells produced by the consec- 

 utive partition of its original cavity, according 

 to the mode described in the last Lecture, or 

 the plant assumes in its ultimate growth a 

 stem or leaf-like fonn, in consequence of the 

 new cells remaining attached and forming a 

 more or less compound structure. These 

 plants, collectively called cellular, are objects 

 of great interest to the naturalist, and furnish 

 examples of vegetable development that are 

 not without their value in the elucidation of 

 that of the higher orders. In the economy of 

 Nature, too, they are agents ever active and 

 efficient. The sea-weed, the lichen, the moss, 

 and the fungus, constitute no unimportant 

 links of the vast chain of organic existence ; 

 but being for the most part beyond the pale 

 of cultivation, any extended noticg of their 

 individual sti'ucture and charactei's would be 

 incompatible with our present subject ; and I 

 shall therefore now only observe that, with 

 the exception of the last, subsisting chiefly, 

 if not solely, by absorption over their whole 

 surface from the surrounding medium, the vi- 

 tal action is less complicated than in those in 

 which the necessary food imbibed by one set 

 of organs, requires conveyance to others be- 

 fore it can become assimilated or converted 

 into the growing substance of the plant. 



The Lycopodums, or club-mosses, the 

 Eqidsctums, or horse-tails, and the ferns, con- 

 Btitute a scries occupying a middle station be- 

 tween the cellular and the flowering plants ; 

 commencing, like the former, with a devel- 

 opment simply cellular, they acquire in their 

 ulterior growth the ducts and woody tissue 

 that characterize the latter, to which the pres- 

 ence of stomates in the leaves of the ferns, as 

 shown in our last Lecture, indicates an ap- 

 proach still nearer ; the absence of flowers 

 and of distinct spiral vessels being the princi- 

 pal structural diiOFerences. 



The influence, direct and indirect, of these 

 flowerless plants upon the nobler tribes of 

 vegetables is at present little appreciated, or 

 at most only by the philosopher in his silent 

 researches into the secrets of Nature ; there 

 are still, however, many points in their histo- 

 ry intimately connected with the interests of 

 the practical agriculturist, and that may here- 

 after come imder our discussion, although we 

 must leave them for a time, to pursue that of 

 the stmcture of their more immediately im- 

 portant associates, the flowering plants. 



Although composed of elementary organs 

 of the same denomination, and apparently 

 agreeing in the general phenomena of their 

 growth, so far as previously detailed, flower- 

 ing plants are distinguishable into two groups 

 or classes, diSering in vaiious featm-es, but 

 especially in the structure of theu* stems or 

 ti-unks; the tissue or organic substance of 

 which is either chiefly disposed in concentiic 

 masses, oris more or less mingled throughout, 

 the tubular with the cellulai- portion ; and 

 from the mode in which its increase takes 

 place under these different circumstances, the 

 former are generally said to be exogens or 

 " outward growers," the latter endogens or 

 " inward growers." The exogens form by 

 far the larger, and perhaps the more impor- 

 tant of these two classes in regard to human 

 economy in the aggregate ; but the endogens 

 are far from being inferior to them in one re- 

 spect, and that the most valuable of all— the 

 production of food. 



Fie. 1. 



SECTION OF EXOGENOUS STEM. 



* Continued from poge 348, vol. iii. Monthly Journal of Aniculture, 

 (763) 



