18 



MORPHOLOGICAL DEVELOPMENT. 



altered ; in the higher types, the cells are so fused together 

 as to constitute cylinders divided by septa. Here, how 

 ever, the indefiniteness is still great : there are no specific 

 limits to the length of any thread thus produced ; and none 

 of that differentiation of parts required to give a decided in 

 dividuality to the whole. 



To constitute something like a true aggregate of the 

 second order, capable of serving as a compound unit, that 

 may be combined with others like itself into still higher 

 aggregates, there must exist both mass and definiteness. 



183. An approach towards plants which unite these cha 

 racters, may be traced in such forms as Bangia ciliaris, 

 Fig. 24. The multiplication of cells here takes place, not in 

 a longitudinal direction only, but also in a 

 transverse direction ; and the transverse 

 multiplication being greater towards the 

 middle of the frond, there results a differ 

 ence between the middle and the two ex 

 tremities a character which, in a feeble 

 way, unites all the parts into a whole. Even 

 this slight individuatioii is, however, very 

 indefinitely marked ; since, as shown by 

 the figures, the lateral multiplication of cells 

 does not go on in a precise manner. 



From some such type as this there appear 

 to arise, by slight differences in the modes of 

 growth, two closely-allied groups of plants, 

 having individualities somewhat more pro 

 nounced. If, while the cells multiply lon 

 gitudinally, their lateral multiplication goes on in one direc 

 tion only, there results a flat surface, as in Ulca linza, Fig. 

 25 ; or where the lateral multiplication is less uniform in its 

 rate, in types like Fig. 26. But where the lateral multipli 

 cation occurs in two directions transverse to one another, 

 a hollow frond may be produced sometimes irregularly 



