26 



MORPHOLOGICAL DEVELOPMENT. 



the indefiniteness is still great. There are no specific limits 

 to the length of any thread thus produced, and there is 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 

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

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

 24r 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 individuation 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 ap- 

 pear to arise, through slight differences in 

 the modes of growth, two closely-allied 

 groups of plants, having individualities 

 somewhat more pronounced. If, while 

 the cells multiply longitudinally, their lateral multiplication 

 goes on in one direction only, there results a flat surface, as 

 in the genus Ulva (Sea-lettuce) or in the upper part of the 

 thallus of E nter omorplia Linza, Fig. 25 ; or where the lateral 

 multiplication is less uniform in its rate, in types like 

 Fig. 26. But where the lateral multiplication occurs in two 

 directions transverse to one another, a hollow frond may be 

 produced — sometimes irregularly spheroidal, and sometimes 



