EXOGENS. 235 



Class VII. EXOGENS. 



Dicotylodones, Juss. Gen. 70. (1789) ; Desf. Mem. Inst. 1. 478. (179G).— ExorhizeJE and Synorhizeae, 

 Rich. Anal. (1808j.— Dicotyledoneae or Exogense, DC. Theor. p. 209. (1813).— Plianerocotyledonea; 

 or Seminiferse, Ai/ardh. Aph. 74. (1821).— Anthophytse and Carpophyta?, Oken. — Dichorgana, 

 )ScAm^;^.— Phylloblastae, Reichcnbach. —llomogeus, Lindl. in But. Rey, lfci39.— Acramphibrya, Endl. 

 Gen. p. 258. (.1837). — Synechophyta, Schlciden. 



By common consent the plants to which botanists formerly gave the name 

 of Dicotyledons, and which now bear that of Exogens, are recognised as tlie 

 most completely formed of all the Vegetable Kingdom. In the more highly 

 organised species they possess a degree of vitality unknown except among 

 Gymnogens. A century or two terminates the life of an Endogenous tree, 

 unless in a few rare cases ; while many Exogens may have been the 

 monarchs of their forests even at the commencement of the Christian era. 

 This arises from their peculiar manner of growth, which insures a renova- 

 tion of their vigom' with each succeeding year ; and it is in allusion to this 

 circumstance that their name has been contrived. 



Exogens, or outward growers, are so called because, as long as they con- 

 tinue to grow they add new wood to the outside of that formed in the pre- 

 vious year ; in which respect they differ essentially from Endogens, whose 

 wood is constructed by successive augmentations from the inside. All the 

 trees of cold chmates, and the principal part of those in hot latitudes, are 

 exogenous. In an Exogen of ordinary structure the embryo consists of a 

 cellular mass, in which there is usually no trace of woody or vascular tissue ; 

 but as soon as germination commences fine ligneous cords are seen proceed- 

 ing from the cotyledons towards the radicle meeting in the centre of the 

 embryo, and forming a thi'ead-like axis for the root. As the parts grow the 

 ligneous cords are increased in thickness and number, and having been in- 

 troduced among the cellular mass of the embryo, are separated from each 

 other by a portion of the cellular substance, which continues to augment 

 both in length and breadth as the woody cords extend. By degrees the 

 j)lumule or rudimentary stem becomes organised, and having lengthened a 

 little, forms upon its surface one, two, or more true leaves, which gradually 

 expand into thin plates of cellular substance traversed by ligneous cords or 

 veins converging at the point of origin of the leaves. If at that time the 

 interior of the young plant is again examined, it will be found that more 

 ligneous cords have been added from the base of the new leaves down to the 

 cotyledons, where they have formed a junction with the first wood, and have 

 served to thicken the woody matter developed upon the first growth. Those 

 ligneous cords which proceed from the base of the leaves do not unite in the 

 centre of the new stem, there forming a solid axis, but pass down parallel 

 with the outside, and leave a small space of cellular tissue in the middle ; 

 they themselves being collected into a hollow cylinder, and not uniting in 

 the middle until they reach that point where the woody cords of the cotyle- 

 dons meet in order to form the solid centre of the root. Subsequently the stem 

 goes on lengthening and forming new leaves : from each leaf may again be 

 traced a formation of woody matter disposed concentrically as before, and 

 uniting with that previously formed : a cylinder of cellular substance being 

 always left in the middle. The solid woody centre of the root proceeds 

 in its growth in a corresponding ratio, lengthening as the stem lengthens, 

 and increasing in diameter as the leaves unfold and new woody matter is 

 produced. The result of this is, that when the young Exogen has arrived 

 at the end of its first year's growth it has a root with a soUd woody axis, 



