BOTANY. XI 



The stem does not always stand in the air [aerial) ; it is sometimes beloAV 

 ground (subterranean). The hxtter are sometimes called roots ; true roots, 

 however, difler from these, in never possessing scales (rudimentary leaves) 

 or nodes, from whicli may be developed e^'^es (rudimentary buds). Tlie 

 croivn of the root is a shortened stem, often partly subterranean, and remain- 

 ing in some plants after the upper portions have withered. This it is wliich 

 constitutes the persistent portion of the ascending axes of the perennial 

 plant. Here the internodes are very short, and the nodes crowd so closely 

 together that there appears to be no stem. A rhizome or root-stock is a 

 stem running along the surface of the ground, partially covered with soil, 

 and sending out leaf buds from the upper side, and roots from the lower. 

 This is seen in ferns, iris, vfcc. A pseudo-bulb is an enlarged bulbose aerial 

 stem, and is succulent, sometimes with numerous spiral cells and vessels, and 

 a thick epidermis. A soboles is a creeping subterranean stem, sending 

 roots from one part, and leaf buds from another. A tuber is produced by a 

 swelling of the internodes, caused generally by a deposit of starchy matter, 

 as in the potatoe. The eyes of the potatoe are the leaf buds on the 

 abbreviated and highly expanded stem. A conn is a solid underground 

 stem, which, of a roundish form, neither creeps nor roots, and is invested 

 by series of imbricated scales, as in the tulip. It developes a second corm to 

 one side, which feeds on the first and destroys it. itself to be devoured in turn 

 by a successor. 



Stems, with respect to their structure, are either exogenous, endogenous, 

 or acrogenous. Exogenous stems (exogens) are those Avhich increase inde- 

 finitely by layers applied to the outside. Stems are endogenous (endogens) 

 when the bundles of vascular tissue are produced in definite fasciculi, and 

 converged towards the interior, all additions being made in the interior. In 

 the acrogenous stem (acrogens) the vascular bundles are all developed simul- 

 taneously, and not in succession, the elongation of the stem depending on the 

 union of the basis of the leaves or the petioles, and the extension of the 

 growing point or summit. In addition to the above, we have thallogenous 

 plants {thallogens) Avhere there is simple elongation or dilatation, without leaf 

 buds or leaves, and dictyogeus, Avhere the stem has the structure of endogens, 

 and the roots nearly that of exogens, as in Smilax. We shall now refer to these 

 more particularl}^ ; premising, however, that there are modifications of the 

 embryo which run parallel to those of the stem, the exogen having a germ with 

 two seed lobes or cotyledons (hence dicot^dedonous) ; the endogen, one with 

 but a single cotyledon (monocotyledonous) ; and no lobe whatever in the acro- 

 gen (acotyledonous). 



In the exogenous or dicotyledonous stem, we have the type of most trees 

 of temperate climates, embracing both a cellular and vascular system. The 

 cellular system includes the outer bark, the medullary rays, and the })ith ; the 

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

 the earlier stages of growth the young exogen is almt)st entirely cellular ; 

 after a time, however, we perceive wedge-shaped bundles, edges of whicli 

 point towards a common centre, arranged around a central cellular mass 

 called liitli, which is connected with the outer bark by means of cellular 



11 



