122 MORPHOLOGICAL DEVELOPMENT. 



serve, next, that the part which lifts itself into the water 01 

 air, is more or less decidedly radial. Each upward growing 

 tubule of C odium aclhcercns, Fig. 4, has its parts disposed 

 with some regularity around its axis ; the upper stem and 

 spore-vessel of Uydrogastrum, Fig. 5, display a lateral 

 growth that is approximately equal in every direction ; and 

 the branches of the Botrytls, Fig. 6, shoot out with an ap- 

 proach to evenness on all sides. Plants of this low type 

 are naturally very variable in their modes of growth : each 

 individual being greatly modified in form by its special cir- 

 cumstances. But they nevertheless show us a general like- 

 ness between parts exposed to like forces, as well as a general 

 unlikeness between parts exposed to unlike forces. 



Respecting the forms of these aggregates of the first order, 

 it has only to be added that they are asymmetrical where 

 there is total irregularity in the incidence of forces. We 

 have an example in the indefinitely contorted and branched 

 shape of a fungus-cell, growing as a mycelium among the 

 particles of soil or through the interstices of organic tissue. 



§ 218. Re-illustrations of the general truths which the 

 forms of these vegetal aggregates of the first order display, 

 are furnished by vegetal aggregates of the second order. 

 The equalities and inequalities of growth in different direc- 

 tions, prove to be similarly related to the equalities and in- 

 equalities of environing actions in different directions. 



Of spherical symmetry, an instance occurs in the Volvoos 

 globaior. The ciliated cells, here so united as to produce a 

 small, mulberry-shaped, hollow ball, cause, by the movements 

 of their cilia, a simultaneous rotation of the ball and pro- 

 gress of it through the water. There is nothing to de- 

 termine the axis of rotation or the direction of rotation. 

 And if the axis and direction of rotation continually vary, 

 as we may conclude that they do, then the different mem- 

 bers of the aggregate severally occupy in their turns like 

 positions towards surrounding agencies; and so are not 



