346 ALTERNATION OF GENERATIONS 



the peculiarities of the generation which arises from a sexually 

 formed cell 



Protoplasm has so far proved much too complex for an analy- 

 sis into the structures which determine its qualities and possi- 

 bilities of development; that is, we do not know why the egg 

 of a fern develops into a fern and that of a club moss into a 

 club moss; both are cells with a general similarity of cell struc- 

 ture. But the possibilities of fern and club moss are nevertheless 

 present in the respective eggs, and the one could not possibly be 

 made to produce the other plant. It is generally believed that 

 the characteristics of eggs are determined by the structure of 

 their protoplasm, represented perhaps by means of the invisible 

 molecules and groups of molecules in its chemical and physical 

 composition. The structures that are assumed to give distinct 

 character or possibilities of development to protoplasm are called 

 rudiments. 



It is doubtful whether we shall ever be able to distinguish 

 the rudiments, but there are some larger structures in the cell 

 which with care can be followed through the cell divisions from 

 generation to generation. The most interesting of these are 

 the chromosomes, which are very characteristic structures most 

 clearly seen during the processes of nuclear division (Sec. 

 199). The substance of the chromosomes, called chromatin, 

 is the most important material in the nucleus. Chromatin can 

 be deeply colored or stained in thin sections of tissue after 

 special methods of treatment. It is present in the resting 

 nucleus, generally in the form of an irregular network. The 

 chromosomes are formed from the chromatin and appear during 

 the early stages of nuclear division. Each chromosome then 

 divides into halves, and the two sets of daughter chromosomes 

 are distributed with each nuclear division. 



It is an important fact that the number of chromosomes for 

 the nuclei of each plant is fixed, and the number is usually 

 not very large. Thus the gametophytes of a red alga (Poly- 

 siphonia, Sec. 245) have about 20 chromosomes, but those of 



