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 
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