i-o 



EXTERNAL CONFORMATION OF PLANTS. 



knowledge of the peculiar'properties of this ideal line may under these circumstances 

 be of great use to morphology. In some cases it may be applied with advantage even 

 to the relative position of whorls. But in a large number of cases other constructions 

 appear much more natural, since they afford an easier explanation of the relative 

 positions, and are more in accordance with the phenomena of growth. The 

 construction of the desired genetic spiral is altogether impossible where the leaves 

 are formed in simultaneous whorls ^ as the petals, stamens, and carpels of most 

 flowers, or even in successive whorjs where the members proceed from one 

 point of the axis, and are formed in advancing order right and left, as in Characeae 

 and the flowers of Reseda (Fig. 137). In the successive whorls of Salvinia natans 

 the construction of a genetic spiral would be equally impossible. Fig. 142, B, 



shows the diagram of the stem 

 ^ of this plant with three con- 



secutive three-leaved whorls ; 

 in each of these the leaf iv is 

 formed first, then the leaf Z^, 

 and finally the leaf Z.^. If 

 an attempt be made to con- 

 struct the spiral, it must pass 

 from w over L^ across to Z^, 

 then again in the same direc- 

 tion over IV across to L^ ; 

 the figure thus formed is 

 a circle, in which the diver- 

 gences of successive leaves are 

 very difi'erent. If we now pass 

 to the next whorl, the line pro- 

 ceeds in a spiral direction to 

 the next leaf w \ but then, to 

 retain the genetic succession 

 in the second whorl, the line must be continued in an opposite direction ; and 

 this is repeated with every new whorl. It is evident that no clear conception 

 can be obtained in this forced manner, and the whole construction appears alto- 

 gether superfluous, since it is required by no feature in the history of develop- 

 ment. The stem of this plant is constructed, as Pringsheim has .shown, of two 

 rows of segments (G, H, J, K, &c., in Fig. 142, A)^ which arise alternately right 

 and left fro*m the apical cell. Even before the formation of the leaves each segment 

 undergoes various divisions, and in this manner sections of the stem are formed 

 which alternately perform the functions of nodes and internodes of the stem. 

 Each nodal section consists of the anterior half of an older segment and the posterior 

 half of a segment next younger iK age, as shown in the figure. An internode is 

 formed of a whole segment of one row and of two half-segments of the other 



Fig. H2.—A the veijetative cone of the stem o( Sa/vinm natajis, regarded 

 diagranimatically and looked at from above; xx projection of the plane which 

 divides it vertically into a right and left half; the segments are indicated by 

 stronger outlines, their divisions by weaker lines; the succession of the segments 

 is denoted by the letters F—P; B diagram of the stem with three whorls of leaves, 

 its ventral side indicated by vv; w the first-formed floating leaf; L\ the aerial 

 leaf formed next; /,•> the second aerial leaf of the same whorl formed last of all 

 between the two first (after Pringsheim). 



' Many writers employ even in such cases the conceptions borrowed from a spiral arrangement, 

 considering arbitrarily as of successive origin the members of the whorl which arise simultaneously, 

 by which means the path to more accurate knowledge is stopped. 



