18 



MORPHOLOGICAL DEVELOPMENT. 



altered ; in tlie higlier types, the cells are so fused togetlicr 

 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 somethino: like a true ao^o^reo^ate 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 Baiigia ciliarisj 

 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 Avhich, in a feeble 

 way, unites all the parts into a whole. Even 

 this slight individuation is, however, very 

 M indefinitely marked ; since, as shown by 

 § the figures, the lateral multiplication of cells 

 H does not go on in a precise manner. 



From some such tj^e 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 Ulva 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, 

 H hollow frond may be produced — sometimes irregularly 



