STRUOTUKE OF STEMS. 49 



Gladiolus. A Corm'is only of one year's duration, while a rhizome or 

 root-stock consists of a string of annual growths, persistently con- 

 nected. It is distinguished from a root by sending off buds annually 

 in the form of small corms or thickened branches, either from the 

 apex, as in Gladiolus, or from the side, as Colchicum (fig. 110 a"). 

 These buds feed on the original corm a', and absorb it. In the 

 Crocus, after flowering, may be seen the withered parent corm ; new 

 corms, which are in reality the basis of the flowering axis, branching 

 from the old corm ; and in the axil of the leaves of the flowering stem 

 small buds ready for another season. In Colchicum autumnale 

 (Meadow Safii-on), we find in autumn the flowering stem united to 

 the side of the corm at its base. The two lowest sheaths of the 

 flowering stem produce buds in their axils. The flowering stem 

 withers, and the internodes between the two buds form a new corm, 

 while the old one decays. 



Internal Structure of Stems, 



Stems, according to their structure, have been divided into three 

 classes : — Exogenous (s^ti), outward, and yivmiiv, to produce), when the 

 bundles of vascular tissue are produced regularly in succession exter- 

 nally, and go on increasing indefinitely in an outward direction. 

 Endogenous (hdov, within), when the bundles of vascular tissue are 

 produced in definite bundles and converge towards the interior, addi- 

 tions being thus in the first instance made internally. Acrogenous 

 {&x^og, summit), when the vascular bundles are developed at the same 

 time and not in succession, the- addition to the stem- depending on the 

 extension of the growing point or summit. The plants which exhibit 

 these three kinds of stem are distinguished also by the structure of 

 their embryo. Thus exogenous stems are met with in plants having 

 an embryo or germ which has two cotyledons or seed-lobes, hence they 

 are called Dicotyledonous (dig, twice, and xorvXr^uv, a seed-lobe); plants 

 with endogenous stems have only one cotyledon, and are called Mono- 

 cotyledonous (//^ovog, one) ; while plants with acrogenous stems have no 

 cotyledons, and are called Acotyledonous (a, privative). The terms 

 connected with the embryo- -will. be afterwards fully explained. 



Exogenous or Dicotyledonous Stem. 



The Exogenous or Dicotyledonous stem characterises the trees of 

 this country. It consists of a cellular and vascular system ; the for- 

 mer including the outer bark, medullary rays, and pith ; the latter, 

 the inner bark, woody layers, and medullary sheath. In the early 

 stage of growth the young dicotyledonous stem is entirely cellular ; 

 but ere long fusiform tubes appear, forming bundles, having the 



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