156 
ROYAL HOETICULTrRAL SOCIETY. 
variety of conditions are thus required to determine this point. In 
any case, however, it is desirable to record the facts observed. 
Certain effects on particular plants, which in all probability 
were due to the drought, are mentioned under each plant. Suffice 
it to say, by way of summary, that they all suffered more or less, 
and this was especially observable after the cutting. Some died 
after this ; others recovei-ed but slowly, The grasses, trefoils and 
Lotus suffered severely, Achillea and Carum dropped their flowers 
before the seed was ripe. 
At the conclusion of the previous Eeport allusion was made to 
the " habit " or special organization of the plant as an element of 
the very highest importance in all questions concerning growth ; 
and the result of the observations in 1870 does but increase my 
sense of the importance of this factor. In no single instance, 
either in 1869 or 1870, was any absolute change in habit observable. 
Changes in degree there were, a certain peculiarity being intensi- 
fied, or the reverse, according to varying conditions of manuring ; 
but there was nothing that would lead one to infer that by the 
agency of any manure, or the operation of any climatal vicissitudes* 
the stronger-growing ones would not only destroy our crops, but also other 
weeds less vigorous than themselves. The facts thus presented to our notice 
by Professor Hoffmann are quite in accordance with other observations on this 
interesting subject ; but they are not sufficient, as M. Duchartre well remarks, to 
explain all the conditions of this complicated problem : thus, in the district ad- 
joining the locality where Professor Hoffmann's experiments were carried on, the 
predominant plants are not the same as those which ultimately proved victors in 
the experimental beds. 
"Again, there is little in the organization of such a plant as the American water- 
weed {Elodea canadensis) to account for its predominance over the Potamoge- 
tons and other water-weeds of our ditches, or in the nature of the common 
Watercress, which is becoming so great a nuisance in the watercourses of New 
Zealand — phenomena the more curious when it is remembered that in America 
the Elodea (Anacharis) is no more of a nuisance than is the Watercress 
here." 
Note added as this sheet is passing through the press, October 1872. — The 
general results of Prof. Hoffmann's experiments are given in the Botanische 
Zeitung, July 19, 1872. The Professor regards variation as the manifestation 
of an innate tendency. The influence of external conditions depends on spon- 
taneous variation and adaptation, or natural selection of the form best adapted 
to the particular circumstances. But, on the other hand, Professor Hoffmann 
considers the fixation of varieties or the filiation of species by descent to demand 
further demonstration. 
