DIRECTIONS OF GROWTH, 20J 



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 different 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 different 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 

 Fucaceae and Metzgeria), stems {Marchantia, Selaginella ^), and leaves (in some Ferns). 

 A different development then generally t^kes 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 

 different planes ; and this is also the case with the roots of Lycopodiaceae (see Nageli 

 and Leitgeb, and Pfeffer, I.e. 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 



I is directly brought about by external conditions, such as pressure, gravitation, and 



I 



* In the roots of Selaginella, however, the successive bifurcations lie in planes at right angles. 

 ' This subject has been treated by Hofmeister (AUgemeine Morph. §§ 23, 74) from another 



