40 
THE ROOT 
adventitious roots of the Palms are homologous with those 
of the Podostemaceae. 
When they arise below ground, as in grasses, bulbs, &c., 
these roots generally have the structure and function of true 
roots, but when they arise above ground ( aerial roots ) they 
often perform new functions and have other structure. 
Subterranean adventitious roots may be in tufts of uniform length, 
as in grasses {fibrous roots) ; tuberous for storing reserves (Ranuncu- 
laceae, Orchis) ; parasitic or saprophytic (Chap. III.) ; or may give off 
subaerial aerating branches (mangroves, Chap. III., and cf. Rumex, 
Jussieua, Sesbania). Aerial roots may form buttresses (Palmae, Panda- 
naceae, Rhizophora, &c.) to support the stem, as well as absorbent 
organs ; pillars (Araceae, Ficus, &c.) ; clasping and climbing organs 
(Araceae, Hedera, Tecoma, Orchidaceae) ; water absorbing organs 
(Orchidaceae, Velloziaceae) ; flattened green assimilating organs (Podo- 
stemaceae, some Orchidaceae) ; thorns (Acanthorhiza, &c.) ; parasitic 
suckers (Cuscuta, Viscum, &c.), &c. See Chap. III. and Part II. 
The Shoot. The primary shoot developes from the 
upper end of the seedling (cf. fig. i, p. 37). The plumule 
or bud soon grows onwards and gives rise to a stem bearing 
leaves at points called nodes , the spaces between being 
internodes . Branches often form in the upper angles or 
axils between the leaves and stem. The advantages of 
this differentiation of the shoot into stem and leaf are great ; 
the leaves require plenty of light and air to carry on assimi- 
lation. The stem serves as a support to spread them out 
for this purpose and to carry water to them, while they 
themselves are thin, exposing a large surface to the air and 
light. Often the shoot or part of it is used as a reservoir 
for storage of reserves. Living as it does under every 
variety of position, climate, and environment, the shoot 
is far more varied in form than the root. 
The symmetry of the mature shoot is very varied, and 
the orientation of different shoots and organs to the stimuli 
of light and gravity almost equally so. The ordinary erect 
stem is heliotropic, and negatively geotropic ; its branches 
and leaves, as well as creeping stems, generally plagiotropic 
to these stimuli. 
The Bud which crowns the tip of the stem is simply a 
much-condensed shoot, a short stem with crowded nodes, 
at which are borne young leaves. The tip of the stem 
