MATURATION 161 



we might say that the somatic or vegetative cells and the germ cells all 

 have the haploid chromosome group. In fertilization the diploid group 

 is formed, but is then retained through only two generations, after which 

 the haploid condition is restored. In other words the predominating 

 stage in the life cycle is that with the haploid chromosome group, the 

 diploid group occurring only in the divisions following fertilization; 

 "haploid" is here synonymous with "somatic." 



In many other plants, such as the ferns and mosses among others, the 

 life cycle is more equally divided into two distinct periods, one carried 

 on with the haploid (in the usual sense of the word), one with the diploid 

 chromosome group. The cells of the fern while it is in the typical 

 "fern-plant" stage have the diploid group, but during the formation 

 of spores by this plant, reduction occurs, the reduced number appearing 

 in the spore mother cell. And in all of the cells of the prothallus, 

 derived from the spore, the haploid number remains; no further reduc- 

 tion occurs when the prothallus forms gametes. The diploid number is 

 only restored by the union of two gametes in the formation of the new 

 fern plant, throughout the existence of which it is retained. It is a 

 matter of considerable theoretical interest that the familiar alternation 

 between the sporophyte and gametophyte generations, between fern 

 plant and prothallus, for example, should be accompanied by a corre- 

 sponding alternation between the diploid and haploid chromosome 

 groups. We may relate this to the condition in the green Algae by saying 

 that the number of cell generations following fertilization, in which the 

 diploid chromosomes are retained, is greatly increased and the number 

 with the haploid group correspondingly diminished, indeed in most 

 cases here, the diploid stage is of greater duration than the haploid. In 

 the higher plants (Gymnosperms and Angiosperms) it is agreed that the 

 prothallus, i.e., the stage with the haploid chromosome group, is repre- 

 sented only by certain vestiges the pollen tube and embryo sac, and it 

 is significant that here, after the two maturation divisions leading to the 

 formation of the germ cells, two or more (but never more than a few) 

 additional divisions occur giving rise to these vestiges; the haploid 

 chromosome number is found in all of these divisions. Here then the 

 phase with the reduced number of chromosomes is still more limited 

 practically to the extent found in animals. And whereas in the lower 

 plants the diploid stage is restricted to two cell generations, in the higher 

 plants it is the haploid stage which comes to be so limited. 



Many consider the gametophyte generation, i.e., the prothallus, or 

 its equivalent in other forms, as the primary form or phase ; consequently 

 they regard the number of chromosomes in the cells of this phase, the 

 haploid number, as primitive or normal, and not as a reduced number. 

 Correspondingly the diploid number would result from a doubling, not 

 from a restoring of the normal. The development of this point of view 



