STEMS OF MONOCOTYLEDONS AND DICOTYLEDONS. 129 



the centre of the stem, and that the hard and thick vascular 

 bundles, situated at the periphery of the stem, are older than 

 the softer ones occupying the centre. For stems like those of 

 Palms he used the term endor/enoics, giving the name exogenous 

 to the other class, in which new layers are added to the outside 

 of the wood. The terms endogenous and exogenous were 

 adopted b3' De Candolle, and have played an important part 

 in Systematic Botany. Comparative researches have shown that 

 the term endogenous as applied to the growth of stems like 

 those of Palms is not appropriate, and hence the correlative 

 words have been generally 

 abandoned as names of the 

 two great groups of plants. 

 They are now generally re- 

 placed by tl)e words monocotj'- 

 ledonous and dicotj'ledonous 

 (see Vol. I. p. 69). 



Moreover, it is now gener- 

 ally admitted that, although 

 the distinctions pointed out in 

 366 — namely, those relating 

 to the arrangement and course 

 of the bundles — are valid for 

 most plants of the two great 

 groups, monocotyledons aud 

 dicotyledons, they do not hold 

 for all. 



380. Instead of describing 

 the numerous exceptions to 

 both of these groups as ex- 

 ceptions, many authors have 

 endeavored to construct some 

 new classification which shall 

 embrace most of the anoma- 

 lies in one or more co-ordinate divisions. Of these attempted 



tendrils is resumed. Every leaf has five fibro-vascular bundles, which are 

 arranged unsymmeti'ically, as shown in the figure. The long distance through 

 which some bundles can nm before uniting with any others, and the differences 

 in structure at the successive nodes, are clearly exhibited in the diagram. 



Fig. 105. Diasramuiatic, projection of the disposition of tlie fibro-vascular bundles in 

 Phaseolus vulgaris. This diagram, like Fig. 104, superposes two longitudinal sections, 

 botli seen from tbe axis : ct, h, c, <1 ; /, f/, h, i : I, m,n, o ; q, r, .s, t ; u, i\ w, x ; the succes- 

 sive leaf-traces, each with four fascicles. Of the upper leaf-trace, the first two fascicles, 

 tf, s, are visible, e, fc, fc, p^ fascicles for the three leaves below. (Nageli.) 



