THE GENERAL SHAPES OP PLANTS. 137 



of Botrydium, Fig. 5, display a lateral growth that is approxi- 

 mately equal in every direction; and the stems of the 

 Mucor, Fig. 6, shoot up with an approach to evenness on all 

 sides. Plants of this low type are naturally very variable 

 in their modes of growth : each individual being greatly modi- 

 fied in form by its special circumstances. But they neverthe- 

 less show us a general likeness between parts exposed to like 

 forces, as well as a general unlikeness between parts exposed 

 to unlike forces. 



Eespecting 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. Ee-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 directions, 

 prove to be similarly related to the equalities and inequalities 

 of environing actions in different directions. 



Of spherical symmetry an instance occurs in Eudorina 

 elegans. The ciliated cells are here so united as to produce a 

 small, mulberry-shaped, hollow ball which, being similarly 

 conditioned on all sides, shows no unlikenesses of structure. 

 An allied form, however, Volvox globator, presents a highly 

 instructive, though very trifling, modification. It is not 

 absolutely homogeneous in its structure and is not absolutely 

 homogeneous in its motions. The waving cilia of its compo- 

 nent cells have fallen into such slight heterogeneities of 

 action as to cause rotation in a constant direction; and 

 along with a fixed axis of rotation there has arisen a fixed 

 axis of progression. A concomitant fact is that the cells 

 of the colony exhibit an appreciable differentiation in relation 

 to the fixed axis. There is an incipient divergence from 



