DIFFERENT ORIGIN OF EQUIVALENT MEMBERS. 
171 
nearer it is to the apex. The lateral members which are formed from and suffi- 
ciently near the growing apex of an axial structure are apparently always acropetal ; 
but the order is disturbed when lengthening at the apex ceases and new forma- 
tions occur in the primary meristem below it, as in many flowers and in the 
abnormal inflorescence represented in Fig. 126. 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 formation of lateral members 
out of the growing point occur in nearly all plants, and, by their regular repetition 
at definite points of the growing axis, determine the external form of the plant, 
they may be considered as normal^ in opposition to the adventitious production of 
members which takes place at the older parts of the axial structure at a distance 
from the apex and without definite order. Such new formations are equally adven- 
titious even when they are of 
great importance to the plant 
from a physiological point of view. 
Adventitious shoots are generally 
formed internally by the side of 
the fibro-vascular bundles of the 
branch, leaf, or root ; but it does 
not follow from this that all en- 
dogenous shoots are adventitious. 
All the shoots of Equisetaceae are 
endogenous in their origin ; but 
they are not adventitious, since 
they are produced in the primary 
meristem below the apex of the 
mother-shoot, and in a perfectly 
definite order. It is equally in- 
correct to call all roots adventi- 
tious although they arise in the 
interior of the stem, leaves, or 
roots. They are adventitious 
only when they occur in older parts, and even then not always; when they 
arise close to the growing point of a mother-root or a stem, they are arranged 
in strictly acropetal order, and are for that reason not adventitious. When a 
member has a basal zone of growth, and produces lateral members from it, they may 
be arranged in basipetal order, as the sporangia on the columella of Hymenophyl- 
laceae, according to Mettenius, or the segments of the leaves of Myriophyllum. 
(7) When in the higher plants a new individual is formed destined for per- 
manent and independent growth, a leaf-bearing axis is first constituted, i.e. a 
shoot on which roots, hairs, and lateral shoots subsequently arise. In all vascular 
plants this first shoot arises immediately out of the sexually-produced embryo ; and 
the externally undiff"erentiated embryo must therefore be considered as itself a 
primary axis\ In Muscinese, on the other hand, the sexually-produced embryo is 
Fig. 126 — Median longitudinal section through a young inflorescence of 
the sunflower, the broad axis of which has been injured at the apex s, and has 
in consequence ceased growing. The zone z z commenced instead an inter- 
calary growth, and behaved as an apical region taking the place of the 
summit a s a; the bracts and flowers of this apical portion have in conse- 
quence been formed from above downwards in a centrifugal direction ; while 
before the injury to the apex they were produced at 7i n in the normal 
manner acropetally. The relative position of the bracts and flowers is also the 
reverse in the abnormal part of the inflorescence of what we find normally. 
^ Compare what will be found under Rhizocarpeae and Angiosperms in Book II. 
