THE MORPHOLOGICAL COMPOSITION OF PLANTS. 23 



system of an animal. In these cases we have considerable 

 bulk attained, not by a number of aggregates of the first 

 order being united into an aggregate of the second order, but 

 by the continuous growth of an aggregate of the first order. 



182. The transition to higher forms begins in a very 

 unobtrusive manner. Among these aggregates of the first 

 order, an approach towards that union by which aggregates 

 of the second order are produced, is indicated by mere juxta 

 position. Protophytes multiply rapidly; and their rapid 

 multiplication sometimes causes crowding. When, instead 

 of floating free in the water, they form a thin film on a moist 

 surface, or are imbedded in a common matrix of mucilage; 

 the mechanical obstacles to dispersion result in a kind of 

 feeble integration, vaguely shadowing forth a combined 

 group. Somewhat more definite combination is shown us by 

 such plants as Palmella botryoides. Here the members of a 

 family of cells, arising by the spontaneous fission of a parent- 

 cell, remain united hy slender threads of that jelly-like sub 

 stance which envelops their surfaces. In some Diatomacece 

 several individuals, instead of completely separating, hold 

 together by their angles; and in other Diatomacece, as the 

 Bacittaria, a variable number of units cohere so slightly, that 

 they are continually moving in relation to one another. 



This formation of aggregates of the second order, faintly 

 indicated in feeble and variable unions like the above, may 

 be traced through phases of increasing permanence and de- 

 finiteness, as well as increasing extent. In the yeast-plant, 

 Fig. 7, we have cells which may exist singly, or joined into 

 groups of several ; and which have their shapes scarcely at 

 all modified by their connexion. Among the Desmidiacece, it 

 happens in many cases that the two individuals produced by 

 division of a parent-individual, part as soon as they are fully 

 formed; but in other cases, instead of parting they compose 

 a group of two. Allied kinds show us how, by subsequent 

 fissions of the adherent individuals and their progeny, there 



