206 MORPHOLOGY OF MEMBERS, 



ronment of the plant is shown, for example, in the fact that multilateral shoots 

 usually grow upright, while l;>ilateral shoots generally lie horizontal, the principal 

 section being then vertical. Many bilateral shoots cling on one side to a horizontal, 

 oblique, or vertical support, as the Marchantieae, Jungermannieae, the ivy, &c. ; 

 and in that case the principal section is at right angles to the support. Bilateral 

 Structures, leaves or whole shoots and systems of shoots, generally form their two 

 sides to which the principal section is perpendicular very differently in respect 

 to the external world; so that, in addition to a right and left half (right and left 

 of the principal section), there is also a clear distinction between an upper and 

 under side, an attached and free side, a dark and light side; and it is in this 

 respect that the dependence of lateral arrangement on external conditions is most 

 clearly evident. 



In each special case it must, however, be left for more exact observation to 

 settle how far the position of the principal sections of the members of a plant are 

 determined by internal relationships of growth, or by external influences ^ a question 

 wh'ich can seldom be satisfactorily answered when not decided by experiment. To 

 this end the researches on Marchantia poly?jidrpha, begun by Mirbel in 1835 and 

 carried on with great success by Dr. Pfeffer in 1870 (/. c), are of peculiar interest. 

 Dr. Pfeffer shows that the two flat sides of the gemmae of this liverwort are 

 identical ; i. e. each of the two sides has the power of forming root-hairs when 

 turned downwards. Bilateral arrangement and differentiation of the upper and 

 under sides are first developed in the fiat shoot which springs from the gemma. 

 The side of the shoot exposed to light, whatever its position, becomes under all 

 circumstances the upper side which forms stomata, the dark side becomes the under 

 side which produces root-hairs and leaf-like processes. Even after the lateral 

 shoots have been formed, the two sides of the gemma itself are still alike. Similar 

 relationships may also prevail in the germinating spores of creeping Jungermannieae 

 and in the formation of the prothallium of Ferns ; but on this point more exact 

 researches are still wanting. In Ferns only thus much is known, that (according 

 to Wigand) when the light is stronger from one side, the principal section places 

 itself in the direction of the strongest illumination, while the apex of the axis of 

 growth is turned towards the shade. 



What has now been said will serve as a definition only of the most important ideas, 

 and an illustration of the points of view from which observations must be made. The 

 results obtained by them cannot be given in detail ; and since a definite theory has not 

 yet been elaborated, a more detailed description must deal with numerous peculiari- 

 ties and critical comparisons, for which we have no space. A few important facts may, 

 nevertheless, be briefly mentioned. 



(i) In reference to the Direction of the Axis of Growth, it appears to be the general 

 rule that the origin of a new individual coincides with the commencement of a new 

 direction of growth. This is very strikingly the case in the swarmspores of (Edogonium 

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

 produces them, 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 (see 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 



^ Compare Hofmeister, Allgemeine Morphologic, §§ 23, 24. 



