PROTOPLASM : TUE INSTRUMENT OF EVOLUTION AMONG PLANTS. 81 
PROTOPLASM: THE INSTRUMENT OF EVOLUTION AMONG 
PLANTS. 
Substance of a Lecture at Chiswick Gardens. 
By Rev. Prof. G. Henslow, M.A., V.M.H., &c. 
[June 26, 1900.] 
When we regard the infinite diversity of plant forms in nature, tlie 
subject seems bewildering, but closer inspection shows both resemblances 
and differences ; so that classification becomes possible. 
Systematic botanists have therefore arranged all plants in groups 
within groups ; and we thus have species, genera, and orders. 
Two forces, therefore, seem to have been at work, one of which tends 
to perpetuate the form, so that seedlings grow up into adult plants 
resembling their parents. The other tends to create differences, so that 
we detect variations or slight differences among the individuals of 
any particular species. When the variations seem in the eye of the 
botanist to be sufficiently pronounced, he gives the plant another specific 
name. 
The question arises, How came these two tendencies, viz. to be con- 
stant and yet to vary, in one and the same plant ? 
We saw in the first lecture that the process of fertilisation consisted 
in the chromosomes of one of the nuclei within the pollen-tube uniting 
with those of the egg- or germ-cell nucleus prepared for it in the em- 
bryo-sac. As the protoplasm surrounding the nucleus appears to take 
no part in the process we must conclude that the chromosomes carry 
all the hereditary powers within them ; but how it is done is an in- 
soluble mystery ; and so the offspring resemble their parents, if they be 
two individuals of the same species. H, however, it be a hybrid, i.e, 
the offspring of two unlike species, then — as we know — it may resemble 
both by being intermediate in character ; or either parent may have 
been prepotent in impressing its own features more strongly than the 
other. 
As long as plants live under more or less precisely the same condi- 
tions, they show no signs of changing. This is particularly the case with 
social plants as Bluebells, Heath, Daisies, Pines, Bracken, d-c. Botanists 
have recorded no varieties among these plants, at least in our own 
islands. 
On the other hand, plants which are found in various soils are 
generally much more liable to variations. Thus Sir J. D. Hooker describes 
the Common Knotgrass: — Polygonum avicularc proper; var., P. litto- 
rale, littoral, the passage to P. maritimwn, maritime ; var., agrcstinum, 
a field form ; var., arenastrum, a sand-loving one ; var., microspcruiumj a 
small-seeded form ; var., rurivagum, a wayside form." 
Here we have the clue to variations. They are simply due to 
differences in the soil and surroundings. The *' littoral" and ''mari- 
time " forms have somewhat fleshy leaves ; but this character, as of the 
Samphire, is simply due to the presence of salt, as experiments have 
