DIRECTIONS OF GROWTH. 
207 
axis of growth of the embryo of Ferns and Rhizocarps is distinctly transverse to the axis 
of the archegonium. In Phanerogams the direction of growth of the embryonal stem is 
opposed to that of the ovule; the apex of the young stem is formed in a direction 
different to that of the ovule, and it continues its growth in this direction. The forma- 
tion of the sporocarp of Mosses forms an exception to this manner of growth, if it is 
considered as a new individual, but this appears very questionable ; it grows in the same 
direction as the archegonium, and even in the direction of the axis of the stem when the 
archegonium is apical {i.e. in Acrocarpous Mosses). 
A second remark relates to the fixation of the base of the axis of growth. In all 
lateral members and bifurcations the base is the fixed point at which the branching or 
new formation begins ; and in the new formation of an axis of growth from swarm- 
spores and fertilised oospheres, the growth in a definite direction does not begin until 
some one cell has become fixed. This occurs in all swarmspores, which do not begin 
to grow into tubes and filaments until their hyaline end, the anterior one in the 
swarming, has become fixed, even if only on the surface of the water. The ger- 
minating spore also of Ferns and Equisetaceae puts out at an early stage a root-hair 
which fixes it to the support (the macrospore of Rhizocarps and Selaginelleae does not 
require this in consequence of its weight). In a similar manner also the longitudinal 
growth of the embryo of Phanerogams does not begin until it has become attached at 
its posterior end to the apex of the embryo-sac. The embryo of Vascular Cryptogams 
produced by a sexual .process fixes itself laterally by the portion called the foot into the 
tissue of the proLhallium. 
It is only in some Algae of the simplest structure that there is no attachment of 
a point of the newly constituted plant to an external object (for which purpose any 
portion of the generating body may serve). In this case the opposition of base and 
apex disappears; growth may then produce a uniform arrangement in diflferent and 
even opposite directions ; simple threads result in which an anterior and posterior end 
can no longer be distinguished, as in some Desmidieae and Diatoms, or spherical families 
of cells, like Glceocapsa. 
But when once a fixed point is established as a base, the increase of length takes 
place uniformly in one direction only from it ; i. e. whatever grows in this direction is a 
member of a morphologically definite character. This does not however prevent the 
setting up of a new growth in the opposite direction ; but the member which is formed 
in this direction is of a diflferent nature morphologically ; as occurs, for example, in the 
embryos of Phanerogams, in which, according to J. Hanstein's recent researches, the 
origin of the primary root is in fact such that its longitudinal axis must be considered 
as the prolongation of that of the stem in a posterior direction. 
(2) With reference to the Relations of Symmetry, the most important point is that dicho- 
tomous branching is frequently repeated in one and the same plane in thallomes (as in 
Fucaceas and Met%geria), ■äi^ms {Marchantia, Selaginella'^), 2Lnd leaves (in some Ferns). 
A different development then generally takes place on the two sides of the plane of 
dichotomy, one side of the shoot clinging closely to the ground or to upright bodies (as 
in Hepaticae), or one side turning to the light, the other side to the shade (Selaginelleae) ; 
in such cases the shoots have their greatest diameter in the plane of dichotomy. 
Where no such different development of the two sides occurs, as in Lycopodium (espe- 
cially L. Selago according to Cramer), consecutive bifurcations may take place in 
diflferent planes ; and this is also the case with the roots of Lycopodiaceae (see Nägeli 
and Leitgeb, and Pfeflfer, /. c. p. 97). 
As has already been mentioned, it is usually impossible, without experiment, to 
determine whether the position of the principal section of bilateral shoots and leaves 
is directly brought about by external conditions, such as pressure, gravitation, and 
lights The position of the principal section usually shows a definite relation to the 
^ In the roots of Selaginella, however, the successive bifurcations lie in planes at right angles. 
This subject has been treated by Hofmeister (x\llgemeine Morph, §§ 23, 24) from another 
