1 86 EXTERNAL CONFORMATION OF PLANTS. 



peculiarities and critical explanations, for which we have here no space. A few important 

 facts may, nevertheless, be briefly mentioned in addition. 



(i) In reference to the Direction of the Axis of Gro^jjth, it appears to be the general 

 rule that the origin of a new individual coincides with the beginning of a new direc- 

 tion of growth. This is very strikingly the case in the swarm-spores of Oedogonium 

 (Fig. 4, p. 9), the longitudinal axis of which is transverse to that of the filament which 

 produces it, and becomes the longitudinal axis of the new plant ; and the same is the case 

 with the origin of new filaments of Nostoc and Rivularia (cf. Book II. Algae). In 

 many Cryptogams, researches have not yet been made on this point, or it would carry 

 us too far to mention them. It may be mentioned, merely by way of example, that the 

 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 

 opposite to that of the ovule, and continues its growth in this direction. The formation 

 of the fruit 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 began ; but even in the new formation of an axis of growth from swarm- 

 spores and fertilised ovum-cells, the growth in a definite direction does not begin until a 

 cell has become fixed. This occurs in all swarm-spores, which do not begin to grow up 

 into sacs and filaments until their hyaline end, the anterior one" in the swarming, has 

 become fixed somewhere or other, even if only on the bounding surface of the water. 

 The germinating 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 Selaginellae 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 prothallium. 



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 substance (for which pur- 

 pose any portion of the generating body may serve). In this case the opposition of 

 base and apex disappears ; the 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 

 in round families of cells, like Gloeocapsa. 



But when once a fixed point is established as a base, the increase of length takes 

 place uniformly in one direction only from it ; /. 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 e. g. in the embryos 

 of Phanerogam.s, in which, according to J. Hanstein's recent researches, the primary root 

 in fact originates in such a manner that its longitudinal axis must be considered as the 

 prolongation of that of the stem in a posterior direction^. 



(2) In reference to the Relations of Symmetry, the fact must be mentioned that dicho- 

 tomous branching is frequently repeated in one and the same plane in thallomes (as in 

 Fucaceae and Metzgeria), stems (IMarchantia, Selaginella), and leaves (in some Ferns). 

 A different development then generally takes place on the two sides of the plane of 



' By these observations the doubt expressed by me in the first edition as to the position of the 

 primary root in the embryo is solved. (For further details see Book II. Phanerogams.) 



