XX MULTIPLICATION OF NUCLEUS 211 



sufficiently to take their place as protectors of the growing 

 point. 



The multinucleate condition of the adult internodes is 

 also a result of gradual change. In its young condition an 

 internodal cell has a single rounded nucleus (a, int. nd^, int. 

 nd^), but by the time it is about as long as broad the nucleus 

 has begun to divide (d, int. nd ^ ; c, int. nd 2), and when the 

 length of the cell is equal to about twice its breadth, the 

 nucleus has broken up into numerous fragments (c, int. nd"^, 

 D, int. nd'^), many of them still in active (amitotic) division. 

 This repeated fission of the nucleus reminds us of what 

 was found to occur in Opalina (p. 123). 



Thus the growth of Nitella like that of Penicillium (p. 

 190), is apical : new cells arise only in the terminal bud, 

 and, after the first formation of nodes, internodes, and 

 leaves, the only change undergone by these parts is an in- 

 crease in size accompanied by a limited differentiation of 

 character. 



A shoot arises by one of the cells in a node sending 

 off a projection distal to a leaf, i.e., in an axil : the process 

 separates from the parent cell and takes on the characters of 

 the apical cell of the main stem, the structure of which is in 

 this way exactly repeated by the shoot. 



The leaves, unlike the branches, are strictly limited in 

 growth. At a very early period the apical cell of a leaf 

 becomes pointed and thick-walled (Fig. 44, e), and after this 

 no increase in the number of cells takes place. 



The rhizoids also arise exclusively from nodal cells : they 

 consist of long filaments (Fig. 44, c), not unlike Mucor- 

 hyphae but occasionally divided by oblique septa into linear 

 aggregates of cells, and increase in length by apical growth. 



The structure of the gonads is peculiar and somewhat 

 complicated. 



p 2 



