VARIOUS ORIGIN OF EQUIVALENT MEMBERS. 1 49 



(3) The origin of lateral members, whether similar or dissimilar to the pro- 

 ducing member, is either exogenous or endogenoics. It is the former when they are 

 formed by lateral outgrowth of a superficial cell or of a mass of cells including the 

 outer layers of tissue, as in the case of all leaves and hairs and most normal leaf- 

 forming shoots. A member is of endogenous origin when it is covered, even when 

 in a rudimentary condition, by a layer of the tissue of the producing member, as 

 in all roots, all lateral shoots of Equisetaceae, and in adventitious buds. 



(4) Lateral members of any kind are almost always formed in considerable 

 numbers on the axial structure which produces them, and even repeatedly one 

 after another, because the producing structure continues to increase in length, 

 and the conditions are repeated along its length for similar equivalent outgrowths. 

 Thus the stem, so long as it continues to grow at the apex, produces leaves, hairs, 

 often even roots, and generally lateral shoots in great numbers, one after another ; 

 roots usually form in succession many lateral roots, branching leaves usually several 

 laciniaj. If the apical growth ceases early, the number of the lateral members is 

 also limited ; thus the short primary stem of Welwitschia mirahilis produces only 

 two leaves. When the increase in length of the stem is very slow, the formation of 

 lateral shoots from it is sometimes altogether suppressed, as in Isoetes, Botrychium, 

 and Ophioglossum. 



(5) An axial structure may produce the lateral members which are equivalent 

 to one another in such a manner that either only one always arises at the same 

 level or several ; in the first case the members formed in succession are termed 

 solitary, in the second case all the similar members arising at one level form a 

 whorl or verticil. Leaves often occur in whorls, shoots less frequently, roots oc- 

 casionally (in the primary roots of Phanerogams). Within the same whorl the 

 members may arise either simultaneously, as the petals and stamens of many 

 flowers, the whorl of foliage-leaves of many Phanerogams; or the members of a 

 whorl may be successive, as those of Characeas and Salvinia. A whorl is a true one 

 when the level of the axial structure is originally such, as occurs in both the last- 

 named plants and in many flowers ; spurious whorls, on the other hand, are such 

 as are formed by displacement and unequal growth of the axis, as in the Equi- 

 setaceae, where leaves, roots, and shoots arise from transverse zones (nodes) which 

 are themselves formed by displacement of three segments of the stem ^ 



(6) Similar and equivalent lateral members usually arise on the common 

 axial structure in acropetal or basifugal order, i. e. the younger a member is the 

 nearer it is to the apex ; counting from below upwards the members occur in 

 the order of their age. The lateral members which are formed from the pimctum 

 vegetationis of an axial structure sufficiently near the growing apex are apparently 

 always acropetal; but the order is disturbed when lengthening at the apex ceases 

 and new formations occur at the primary meristem below, as in many flowers. 

 The lateral members formed at a greater distance from the growing apex of the 

 axial structure are sometimes, but not always, acropetal. Since branching and 



^ The three segments, which form the contour of the stem, stand at first at different heights, 

 but arrange themselves, as Rees has shown, in a transverse zone (node), which forms outwardly a 

 circular cushion, the rudiment of the leaves (cf. Book II. Equisetaceae). 



