DIRECTIONS OF GROWTH. 185 



to Other relationships, is very near to that of bilateral arrangements, and may be 

 termed a Double Bilateral arrangement. In these cases the principal sections of 

 the leaves are also at the same time principal sections of the stem. In Salvinia, 

 Marsilea, Polypodium aiireum, and Pteris aquilina^ on the contrary, the principal 

 sections of the leaves, arranged in straight rows, lie right and left of the single 

 principal section of the bilateral stem, which is in these cases closely connected 

 with the horizontal growth. 



The relationship of lateral arrangement and symmetry to the external envi- 

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

 usually grow upright, while bilateral shoots generally lie horizontally, and in such 

 a manner that the principal section is vertical. Many bilateral shoots cling on 

 one side to a horizontal, oblique, or vertical support, as the Marchantiae, Junger- 

 manniae, 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 stands at right angles 

 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 be- 

 tween 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 enquiry to settle 

 how far the position of the principal sections of the members of a plant are deter- 

 mined by internal relationships of growth, or by external influences \ a question 

 which can seldom be satisfactorily answered when not decided by experiment. 

 To this end the researches onMarchatih'a polymorpha, begun by IMirbel 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 ; /. e. each of the two sides has the power of forming root-hairs when 

 turned downwards or attached to a firm substance. Bilateral arrangement and dif- 

 ferentiation of the upper and under sides are first developed in the flat shoot which 

 springs from the gemma. The side of the shoot exposed to light, whatever its 

 position, is under all circumstances the upper side which forms stomata, the dark 

 side becomes the under side which produces root-hairs and leafy processes. Even 

 after the lateral shoots have been formed, the two sides of the gemma itself are 

 still of equal value. Similar relationships may also prevail in the germinating spores 

 of creeping Jungermanniae 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 

 plane of the principal section falls in the direction of the strongest ray of light, 

 and the axis of growth, with its apex, is turned towards the shade. 



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

 and in 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 by science, a more detailed description must deal with numerous 



Compare Hofmeister, Allgemeine Morphologic, §§ 23, 24. 1S68. 



