VARIOUS ORIGIN OF EQUIVALENT MEMBERS. 



15.3 



(a) The numerical relationship of the lateral shoots to the leaves which are formed 

 along with them on the same axis is so far variable that the number of the former may 

 be either equal or unequal to that of the 

 latter. If the number is unequal, a greater 

 number of leaves than of branchlets usu- 

 ally arises on the same axis; in jNIosses, 

 Ferns, Rhizocarpeae, Gycadeae, and Coni- 

 ferae a much larger number. A branch- 

 let may arise whenever a perfectly defi- 

 nite number of leaves has been formed, 

 as in many Mosses and some Ferns, or the 

 formation of a branchlet results when the 

 increase in length of the primary shoot 

 and the formation of its leaves ceases for 

 a time, as in the genus Abies, and is subse- 

 quently renewed. When the leaves stand 

 in whorls, the number of the lateral shoots 

 may be equal to that of the members of 

 the whorl, as in Equisetaceae, or it may 

 be smaller, as in Characese. Only rarely 

 is the number of branchlets larger than 

 that of the leaves, as in some Monocoty- 

 ledons and Dicotyledons, where two or 

 more lateral buds often arise side by side 

 above a leaf (Fig. 122), or one above 

 another (as in Aristolochia Sipho, Glcdit- 

 schia, Szc). In most Monocotyledons and 

 Dicotyledons the number of the lateral 

 branchlets (with the exception of the 

 flower-shoots) is, at first, equal to that of 

 the leaves ; but usually only a much smaller 

 number attain a higher development. 



(;y) A relationship in position exists be- 

 tween the origin of the leaves and the normal 

 lateral shoots of a common mother-shoot, 

 since a constant arrangement is found of 

 leaves and shoots in each species and often 



in whole classes of plants, the shoots being always produced either below, beside, or 

 above the leaves. The lateral shoots arise below the leaves (according to the acute 

 investigations of Leitgeb^), probably in all Mosses, as well as in the Hepaticae Radula 

 and Lcjeunia; the shoot springs (as shown in Fig. 106, 2) out of the lower part of a 

 segment of the stem the upper part of which has developed into a leaf. In Fontinalis 

 this occurs below the median line (the symmetrically dividing plane) of the leaf, in 

 Sphagnum laterally below one half of the leaf. According to the same observer, the 

 lateral shoots arise in place of a half-leaf beside the remaining half, in many Hepaticae of 

 the section Jungermannieae (Frullania, Madotheca, Mastigobryum, Jungermann'ia tricho- 

 phylla; Leitgeb, Bot. Zeitg. p. 563, 1871). If each tooth in the leaf-sheaths of an Equi- 

 sctum is considered to be a leaf, the buds originate at the side of the leaves and between 

 them, for they break through the leaf-sheaths between the median lines of the teeth. 



Pig. 120 — HquisetHfn arvense ; longitudinal section of an 

 underground bud in March ; ss the apical cell of the stem ; 

 b — 96 its leaves ; A' A'' two endogenous lateral buds exposed by 

 the cut. Tlie youngest rudiments of buds are to be found, 

 however, at b", and they have probably begun to be formed 

 even at a greater height (X50). 



^ Leitgeb, Beitriige zur Entwickelungsgeschichte der Pflanzenorgane in Sitzungsber. der kais. 

 Akad. der Wissen. zu Wien, Bd. 57, 1SO8, and Bd. 59, 1869; and Bot. Zeitg. no. 34, 1871. See also 

 more in detail, Book II. Mosses. 



