Strasburger • The Periodic Reduction of the Number of the Chromosomes 221 



mother continue active; and the be- 

 haviour of hybrids shows pecuharities 

 which are very instructive for the com- 

 prehension of the phenomena of hered- 

 ity in the offspring of legitimate unions. 

 Hybrids may exhibit in all their parts 

 a combination of the characters of the 

 two parents; or they may show this only 

 in certain parts, whilst other parts pre- 

 sent the distinct characters of one or 

 other of the parents; or they, on the 

 whole, resemble one parent more than 

 the other; or, finally, they may alto- 

 gether resemble one of the parents. 

 Naudin has drawn attention to the 

 fact that, in some hybrids, the charac- 

 ters of the two parents, instead of be- 

 ing blended, are manifested in patches; 

 this may occur in all parts of the plant, 

 but it is especially marked in flowers 

 and fruits. In such a case the hybrid is 

 a sort of mosaic made up of portions of 

 the two parents. Millardet has recently 

 given an account of hybrids which are 

 more like, or, in the extreme case, ex- 

 actly resemble either the father or the 

 mother. 



The process of reduction of the 

 number of the chromosomes by half 

 takes place, in the Muscineae, Pteri- 

 dophyta, and Phanerogamia, in the 

 spore-mother-cells, that is, at the close 

 of the generation developed from the 

 fertilized ovum; but in the lower Cryp- 

 togams, where the cell produced by 

 the sexual act does not give rise to a 

 definite organism representing the 

 asexual generation, the reduction prob- 



ably takes place on the germination of 

 this cell. 



The reduction in number of the 

 chromosomes takes place, among the 

 higher plants, in the mother-cells of 

 the spores, and it is consequently these 

 which must be regarded as the first 

 term of the new generation. They as- 

 sert this their true significance in that 

 they usually isolate themselves from 

 cohesion with other cells and become 

 independent, although this independ- 

 ence is only of practical utility in the 

 case of the products of their division, 

 that is, of the spores. Hence the centre 

 of gravity of the developmental proc- 

 esses which take place in both micro- 

 and macro-sporangia of Cryptogams 

 and Phanerogams, does not lie in those 

 cells, cell-rows, or cell-aggregates, 

 which give rise to the sporogenous tis- 

 sue and have been designated 'arche- 

 sporium' by Goebel. The archesporium 

 still belongs to the sexually-developed 

 asexual generation; it is only the spore- 

 mother-cells which initiate the new 

 sexual generation: consequently the 

 presence or absence of a well-defined 

 archesporium is not a matter to which 

 importance should be attached. For 

 the archesporium is merely the meris- 

 matic tissue from which the spore- 

 mother-cells are derived, a tissue which 

 is frequently, but by no means neces- 

 sarily, differentiated from the surround- 

 ing tissues at an early stage; so that its 

 differentiation cannot be of funda- 

 mental importance. 



QUESTIONS 



1. Can you name any plants that repro- 

 duce entirely by asexual means? 



2. What does Dr. Strasburger believe to 

 have been the course of evolution? 

 Did it proceed from asexual to sexual 

 or from sexual to asexual? 



3. Can you think of any advantage in 

 propagating apple trees asexually in- 

 stead of using seeds? 



4. What phenomenon takes place in the 

 chromosomal mechanism preceding 

 the formation of male and female sex 

 cells? 



5. What is the practical advantage of 

 this phenomenon? 



6. Are Strasburger's ideas about the steps 

 leading to the formation of the sexual 

 generation (gametophyte) essentially 



