36 



STRUCTURE OF STEMS. 



is a thickened stem produced by the approximation of the nodes 

 and the swelling of the internodes, as in the potato (fig 92 i). 

 Tubers are sometimes aerial, occupying the place of branches, more 

 especially when the potato has been made to grow in darkness. The 

 eyes of the potato are leaf-buds. The ordinary herbaceous stem of the 

 potato, when cut into slips and planted, some- 

 tunes forms branches from its base which 

 assume the form of tubers. These tubers 

 occasionally become nodulated, or elongated, 

 or curved in various ways. Arrow-root is 

 derived from the scaly tubers of Maranta 

 arundinacea. A Corm is a solid under- 

 ground stem which does not spread by 

 sending out shoots, but remains of a rounded 

 form, and is covered by thin scales on the 

 outside (fig. 93). It occurs hi Colchicum, 

 Tulip, Crocus, and Gladiolus. It is distin- 

 guished from a root by sending off annually 

 buds in the form of small conns or thick- 

 ened branches, either from the apex as in 

 Gladiolus, or from the side as Colchicum 

 (fig. 93 a"). These buds feed on the ori- 

 ginal conn a', and destroy it. It will be noticed afterwards, when leaf- 

 buds and bulbs are considered. 



Internal Structure of Stems. 



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

 classes: Exogenous (!|<a, outward, and ytwiitiv, to produce), when the 

 bundles of vascular tissue are produced regularly in succession exter- 

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

 genous (1*800, within), when the bundles of vascular tissue are produced 

 hi definite bundles and converge towards the interior, additions being 

 thus hi the first instance made internally. Acrogenous (x.(>of, summit), 

 when the vascular bundles are developed at the same time and not hi 

 succession, the addition to the stem depending on the union of the base 

 of the leaves and 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 (8<V> twice, and noTv^nluii, a seed- 

 lobe) ; plants with endogenous stems have only one cotyledon, and are 

 called Monocotyledonous (ftovos, one); while plants with acrogenous stems 



Fig. 93. Corm or under-ground stem of Colchicum autumnale. r. Roots. /, Leal a', Ascend- 

 ing axis of preceding year, withered, a", Axis of the year, a"'. Point where axis of next year 

 would be formed. 



