FORMS OF STEMS. 33 



in certain circumstances, and on that account they have been called 

 Rhizogens (p<ct, a root, and yiwdiiv, to produce). They are conspi- 

 cuous in Willows, the young branches of which form roots very 

 readily when placed in moist soil. Some hairs occurring on the style 

 of plants are called collecting hairs, from the functions which they per- 

 form in taking up the pollen. In the species of Campanula, these hairs 

 are so formed, that after the pollen has been discharged, their upper 

 part is drawn within the lower. In many hairs a circulation of fluids 

 takes place, connected apparently with their nutrition and develop- 

 ment (fig. 86). In the monuiform purple hairs on the stamens of Trades- 

 cantia, or Spiderwort, this movement may be easily seen under the 

 microscope. The subject of the circulation in hairs will be considered 

 under Rotation. 



STEM OR ASCENDING AXIS. 



Forms of Stems. 



65. The stem is that part of a plant which bears the leaves 

 and flowers. It receives the name of Caulis in ordinary herbaceous 

 plants which do not form a woody stem, Truncus in trees, Caudex 

 in shrubs, Culm in grasses, and Stipe in Palms and Ferns. It is not 

 always conspicuous. Plants with a distinct stem are called caules- 

 cent; those in which it is inconspicuous are acaules. Some plants are 

 truly stemless, and consist only of expansions of cellular tissue, called 

 a Thalha, and hence are denominated Thallogens, or Thallophytes (da^og, 

 a frond, yswa.ziv, to produce, QVTOV, a plant). They have no true vas- 

 cular system, but are composed of cells of various sizes, which some- 

 times assume an elongated tubular form, as in Chara. The cells are 

 sometimes united in one or several rows, forming simple filaments, 

 as in Confervas ; or branched and interlaced filaments, as in some 

 Fungi ; or membranous expansions, as in Lichens and sea- weeds. 



66. Stems have usually considerable firmness and solidity, but 

 sometimes they are weak, and either he prostrate on the ground, thus 

 becoming procumbent, or climb on plants and rocks by means of suckers 

 like the Ivy, being then called scandent, or twist round other plants in 

 a spiral manner like Woodbine, becoming volubile. Twining plants turn 

 either from right to left, as the French bean, Convolvulus, Passion- 

 flower, and Dodder; or from left to right, as Honeysuckle, Hop, and 

 Tamus. Bryonia alba twines from right to left, and from left to right, 

 alternately. In warm climates, twining plants (lianas) often form 

 thick woody stems ; while in temperate regions they are generally 

 herbaceous. Exceptions, however, occur in the case of the Clematis, 

 Honeysuckle, and Vine, the woody stems of which have received the 

 name ofSarmentum (sarmentum, a twig, or cutting of a vine). Some stems 



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