May 6, 1886] 
NATURE 7 
a milky juice on being assailed by them. Other plants, 
as some varieties of the willow, have very slippery flower- 
stalks, which the ants cannot passalong. The forms of the 
flower, too, lend themselves to protective purposes: thus 
Antirrhinum and Linaria have a close-shutting corolla, 
which they cannot enter; Codaa is furnished with free 
hairs growing on the corolla, which block the way to the 
nectar, and which are insurmountable by the insects 
Where such means are not found, in some cases a counter- 
attraction is provided to draw the unwelcome visitors to 
parts where their attentions will be harmless: thus 7z- 
patiens has honey-glands on the leaves which are said to 
stop the ants on their way to the flower. 
Other insects than ants are also to be guarded against. 
Many flowers are capable of fertilisation by more than 
one species of insect, but others are especially adapted 
only to one kind. In these the form of the flower, while 
affording facilities for the proper insect to receive its 
pollen upon the proper region of its body, also presents 
obstacles to others which would be useless. The peculiar 
construction of the corolla in such cases serves as a pro- 
tection to both nectar and pollen. This may be carried 
still further, access to the honey by other than the appro- 
priate channel being hindered by chemical means. An 
instance of this is seen in the Alpine varieties of the 
Aconite, which are adapted for fertilisation by bees. 
Instead of the insect inserting its proboscis into the 
flower from the front, so as to make it pass the stamens 
and pistil, one bee (Gombus mastrucatus) bites a hole in 
the back of the hood formed by the sepals, and abstracts 
the honey. The white variety of the flower is unpro- 
tected against the theft, but the other, blue in colour, has 
a nauseous, bitter taste, and so is let alone. 
Besides meeting the attacks of animals in these 
different ways, plants have to cope with other dangers, 
and require for these another system of defences, which 
are more associated with peculiarities of environment. 
They are assailed continually by varying conditions of 
climate and temperature, and have in many cases very 
curious modifications of structure and habit to correspond 
with these. A danger that threatens most plants, except 
in a few regions of the world, is that of having their 
pollen injured by rain. To meet this many varieties of 
form of corolla have been developed. Many have a long 
narrow tubular shape, the claws of the petals cohering 
together, while the free limbs can curve outwards in fine 
weather, but arch over the tube when wet. Others have 
a campanulate form, with the base of the bell upwards, so 
that rain falling on the flower cannot get near the stamens, 
but is shot off as by aroof. In others the stamens are 
covered over by development of another part of the flower, 
as inthe Iris; the filament of the stamen, too, may be 
broad, and bear the anther on its under surface, as in the 
Naiadacee. It is rather curious that flowers that produce 
large quantities of pollen have not such defences against 
this danger as those which form but little, while the most 
complete adaptations are found in the cases of plants that 
inhabit damp climates. 
Many flowers are defended by habit rather than struc- 
ture. In wet weather they do not open their corollas at 
all, and not a few, even in fine weather, keep open for a 
very little while, only a few hours in many cases. 
Besides rain, other meteorological conditions are fraught 
with danger. One of the most commonly occurring is 
frost ; and allied to this is the loss of heat by radiation 
during the night. The power of resistance to these con- 
ditions varies very much, but in many whose constitution 
. makes them peculiarly susceptible to damage thereby 
_ there has been developed the so-called power of sleep. 
_ The term is no doubt a misnomer, but it has been adopted 
_ and associated with certain well-defined movements which 
_ the leaves of the plants perform at the close and at the 
beginning of day. The movements differ very greatly 
‘with different plants, but they bring about such a position 
of the leaves as will protect the upper surface from radia- 
tion. Some of them are of a very complex nature, parti- 
cularly those shown by certain of the Leguminose, which 
have pinnate leaves. It is in this natural order that 
the property of sleep is most prevalent, certain of the 
Oxalidacez and their allies coming next to them. 
A similar mechanism protects very many plants from 
excess of sunlight, which is injurious to the chlorophyll. 
In bright sunshine the leaves assume a position which 
has been called “diurnal sleep.” In it they present their 
edges and not their faces to the light. In other leaves the 
chlorophyll corpuscles themselves move, taking up a posi- 
tion on the lateral walls of the cells rather than on the 
front ones, or so placing themselves that their profile and 
not their surface is exposed to the sun. In some of the 
Alge, as Mesocarpus and Vaucheria, this sensitiveness is 
seen. 
Other protective devices may be seen by studying the 
adaptations of plants to their conditions of life. ‘Thus the 
leaves of submerged plants are preserved from being 
broken by the currents of water by being minutely sub- 
divided, so that they adapt themselves easily to the 
motion, and do not oppose a resistance. Desert 
plants are protected from drought by the development of 
asucculent habit. Aérial parts of plants, again, are pro- 
tected in many cases from becoming moistened by water 
by a deposition in the cuticular layers of the epidermis of 
varying amounts of wax or resin. 
THE ORIGIN OF OUR POTATO 
HE year 1886, by its tercentenary associations, brings 
once before us the subject of the introduction of 
the potato into our islands, but brings it still with most 
of the connected questions unsolved. 
How, and when, and whence it was brought was con- 
sidered by Banks in 1808, and it was by him attention 
was drawn to a manuscript statement in 1693 by Dr. 
Southwold Smith, F.R.S., that his grandfather received it 
from Sir Walter Ralegh, and sent it to Ireland. 
It was considered by Sabine in 1822, when he con- 
cluded a paper before the Royal Horticultural Society 
with the remark, “The introduction of the potato z7¢o 
Virginia is still involved in obscurity.” 
It has been considered by De Candolle in his “ Géogr. 
Bot. Raisonée” in 1855, and more recently in his ‘‘ Origin 
of Cultivated Plants” in 1882. It has also been considered 
by others. While of the old unanswered questions some 
are now regarded as of mere antiquarian interest, there 
are others to which greater importance is attached than 
there ever has been before. 
Among the latter a fresh interest has been given by 
Mr. Baker’s paper before the Linnean Society in January, 
1884, to the old question, was it S. ¢uberosum that was 
introduced from Virginia? The suggestion he, in con- 
junction with Earl Cathcart, has thrown out, that to 
strengthen our cultivated potato against disease we should 
cross with some other species of tuber-bearing Solanum, 
makes it important we should clearly know what is the 
species we have been for 300 years cultivating. There 
are many other questions surrounding the consideration, 
some of which border on that fundamental question, 
What constitutes a species? 
That simple but highly practical method of approach- 
ing the question, “ What is our species?” the method of 
introducing supposed distinct wild species, and watching 
their changes from year to year in cultivation, has not 
yet been followed sufficiently long, nor with a sufficient 
number of such species to effect much more than esta- 
blish well-founded hopes that by it there is much we may 
learn. At present the twenty(?) years’ cultivation of 
S. maglia is the only experiment on which we can rely. 
What conclusions such experiments may eventually lead 
s to it is impossible to predict, but this is certain, that 
