112 PLANT PHYSIOLOGY 



chromosomes do not, however, correspond exactly to the 

 originals, for in the synaptic stage there has been an 

 exchange of some characters. At the next division the 

 nuclear phenomena are like those of the ordinary 

 vegetative division. 



161. These peculiarities of haploid and diploid chro- 

 mosome number, reduction division, and ordinary (so- 

 matic) division of the nuclei, as well as other observed 

 phenomena of heredity, have led most investigators to 

 conclude that the chromosomes are the chief bearers of 

 heredity. In sexual reproduction, then, is found a means 

 of combining in the most complicated ways the minute 

 or larger differences found in the different parents. 



162. Variations. Hardly any two plants are exactly 

 alike. The differences are of two kinds: (1) a response 

 of the plant to slightly or greatly different environ- 

 mental conditions, and (2) a difference in the constitu- 

 tions of the plants that leads them to. respond somewhat 

 differently in morphological or physiological characters 

 when exposed to the same conditions. These latter 

 are the only ones that demand attention here. They 

 may be slight differences that are apparently not inherit- 

 able (i.e. although the somatic or vegetative cells are 

 somewhat different the sexual cells are not so), or there 

 may actually have taken place a change in the constitu- 

 tion of the protoplasm that affects also the reproductive 

 cells, so that the heredity carriers (probably the chromo- 

 somes) are slightly different in the different plants. 



163. Gregor Mendel, in 1866, published a paper in 

 which he pointed out that certain characters that differed 

 in the two parents and that are mutually exclusive 

 (i.e. that allow of no intermediate form) would appear in 

 the second generation in a pure form in some of the 

 plants. This is now explained by the phenomena taking 



