XVI] THE CHROMOSOME-CYCLE 329 



fact testified by innumerable instances that syngamy doubles the number of 

 chromosomes, and initiates a diploid state. It is also a fact of experience 

 that the number is halved on 

 meiosis, which heralds spore- 

 formation, and the establish- 

 ment of a haploid state. Beyond ^^, 

 these facts there seems to be 0'/^:-kA r-- il 

 nothing fixed for plants at y#T"^'^~^ ^ 

 large as regards their life-cycle. r^ ^ ^ 

 Experience shows that where 

 sexuality occurs plants work 

 this chromosome-cycle into 

 their life -history in various 

 ways as regards somatic de- 

 velopment, and the consequent 

 establishment of "generations." 



The facts above detailed for the Fig. 304. AhophUa van Ge7'tii. Primary leaf with Aneura- 

 T-M- 11 ^ • 1 like prothalloid growths shaded so as to distinguish them 



Fihcales show a certam degree f^o/the leaf itself. (After von Goebel.) 

 of latitude even in this group. 



But in theThallophytesthe proportions and even the sequence of the so-called 

 " generations " may show a much greater latitude of difference, as illustrated 

 by those relatively few Algae and Fungi in which the details have as yet 

 been thoroughly worked out. It is still an open question whether in any 

 given phylum of Thallophytes the point at which meiosis takes place in the 

 life-cycle has been fixed throughout Descent or has been moveable. This 

 question is especially a moot point for the Rhodophyceae: and when the 

 Phaeophyceae and Chlorophyceae have been further examined it seems not 

 improbable that like questions may arise also for them. 



The general impression given by the comparison of Thallophytes so far 

 as their analysis has yet gone, in respect of the fit of the chromosome-cycle 

 upon their somatic development, is that there is a high degree of fluctuation. 

 The two phases do not appear to have settled down to any mutual adjustment 

 that has become general. They illustrate a definite alternation in the making 

 rather than as a present actuality. In the Archegoniatae, however, the life- 

 cycle shows as a normal feature so constant an adjustment of the chromosome- 

 cycle to the somatic development that it justifies the recognition of a definite 

 alternation. No one can doubt in them the normal nature of that succession 

 of events which is demonstrated by all their known life-histories. A biological 

 reason for the constancy of that alternation which they show has been sug- 

 gested in an accommodation to sub-aerial conditions during their Descent. 

 It is probably this that has stabilised their life-cycle. The cytologically 

 distinct generations differ widely in form and structure. The more delicate 



