May 6, 1886] 
NATURE 
5 
range in June at Bombay than at Madras. To investigate 
this point it will be desirable to give the comparative 
ranges for the various elements for the various months at 
the two stations. This is done in the following table. 
TABLE III.—Ranges for the Various Months of the 
Diurnal Variations of the Three Elements at the 
Two Stations 
Bombay Madras 
Declina- Hor. Vert. | Declina- Hor. Vert. 
Month tion force force tion force force 
January 5 alo 389 167 gI 1318 3414 
February ... 117 507 120 | 64 1585 3778 
March 263 571 175 | 104 1785 4847 
April... 392 576 216 218 1851 6273 
May ... 486 473 265 243 1522 4198 
June ... 480 434 257 | 260 1214 2017 
july: ese 0 -c- -408 439 263 | 249 1218 2445 
August... 545 423 30m) 273 IIOL 3203 
September... 550 407 365 282 1331 6401 
October . 258 437 213 IIo 1595 5163 
November... 103 414 ot 73 1362 3633 
December... 136 356 89 86 Ill4 2280 
From this table it will be seen that for both stations 
there is a smaller maximum of declination range about 
May or June, and a larger maximum in September, whiie 
the most decided minima are in November and February 
for both stations. Again, there isa maximum of horizontal 
force range for both stations in April, and also in October, 
while the minima are at Bombay in September and 
December, and at Madras in August and December. 
Finally, at Bombay there is a smaller maximum of 
vertical force range in May and a larger in September, 
while at Madras these occur in April and September. 
The most pronounced minimum of vertical force is in 
December for both stations. 
It would thus appear that there is a very striking like- 
ness between the variations of the three elements at the 
two stations, and that, notwithstanding Mr. Pogson’s 
remark about the vertical force instrument, its results do 
not appear to be without value in a comparison of the 
above nature. BALFOUR STEWART 
PLANTS AND THEIR DEFENCES 
CONSTANT struggle for existence, the consequence 
of the enormous increase in the numbers of the 
individuals of almost every species, is the fate of nearly 
every organism, both animal and vegetable. Some have 
to sustain the attacks of others which are directly an- 
tagonistic to them, and which regard them as prey; in 
the case of others the struggle is rather one to live in the 
face of adverse conditions or peculiarities of environ- 
ment, so that the different organisms are not directly 
hostile, but each affects its neighbour injuriously by 
adapting itself more readily to the changing surroundings, 
and so diminishing the other’s power of obtaining nutri- 
ment, sunlight, or whatever other condition may be the 
object of their competition. Thus have been developed 
in the different competitors different features of their 
constitution—many perfecting powers of active assault, 
others facilities for active or passive defence. The last- 
named is particularly the feature found in the vegetable 
kingdom. The want of locomotion prevents any aggressive 
movement of the individual, and hence success in the 
struggle can only be secured by more complete adapta- 
tion to environment than its competitors can show, or by 
protective mechanisms guarding the individual from the 
assaults of organisms inclined to prey upon it. These 
mechanisms exhibit very great variety, and their object 
often seems obscure till they are looked at in the light of | 
the environment of the plant, the conditions ef its life, 
and the enemies against which it has to contend. The 
specially-exposed points of attack are three: the succu- 
lent leaves and shoots or the attractive fruits are assailed 
by animals in search of food ; the honey secreted by the 
flower to allure to it the particular insect adapted to bring 
about properly the process of fertilisation attracts also 
other insects whose presence is useless for such purpose, 
and which therefore are only robbers ; while the fertilising 
pollen is itself the object of desire on the part of others 
which are equally unable to apply it to its legitimate 
purpose. 
The protective mechanisms of plants, therefore, so far 
as they are directed against aggressive animals, are to be 
looked for mainly in the neighbourhood of the young 
growing parts or the reproductive organs. Not exclusively, 
however, but generally the older vegetative parts are 
defended by their own inherent qualities, such as their 
hardness or wiriness, which keep them from being suit- 
able for the food of their assailants. Such young growing 
parts in many plants, particularly those growing in ex- 
posed regions, are plentifully supplied with thorns, spines, 
or prickles, rendering them in many cases extremely 
formidable. The thorns or prickles may be produced on 
almost all the vegetative organs, and may be merely 
epidermal structures, or much stronger in composition, 
containing considerable developments of woody tissue. 
These thorny plants are most noteworthy in desert countries, 
some that are met with there, notably the so-called 
“wait-a-bit ” thorn of Africa, having spines of immense 
length, and being quite impenetrable by man or beast. 
Cases are not of infrequent occurrence where even the 
lion himself is a considerable sufferer by coming into 
collision with this plant. So great is the development of 
the thorny character in this region that Grisebach con- 
nects it particularly with desert exposure and scarcity of 
vegetation. Nor are thorny plants by any means confined 
to such regions—on our own heaths the gorse is a familiar 
plant, and one sufficiently formidable to passers-by ; 
while other spiny Leguminosz, as the wrest-harrow 
(Ononts spinosa), are not infrequent by the wayside. A 
further peculiarity may be noted in connection with these 
plants : often the thorns do not occur above the point 
which is assailable by the animal in its search for food ; 
while, when the shoot has outlived its period of succulent 
condition, and its tissues have become hard and dry, the 
thorns do not persist, being much more numerous when 
the part is young. . 
Nor is this spiny habit confined to shrubs or trees. 
The cactuses, which are so remarkable a feature of the 
vegetation of America, are equally well protected. Their 
surfaces show great variety of development in this par- 
ticular: some have small groups of thick rigid spines, 
others long flexible needles of intense sharpness, pene- 
trating easily the skin of the assailant, and almost 
impossible to extract. 
More formidable defences even than thorns or prickles 
are found in the varieties of stinging hairs borne so 
plentifully on the leaves of many plants, These are 
represented in England by the two species of stinging 
nettle, which are, as every one knows, capable of pro- 
ducing considerable discomfort to the unwary person 
who handles them. These are, however, not worth men- 
tioning by the side of many of their tropical relations. 
The structure of the hair in all these is similar: a mass 
of cells forms a kind of swollen cushion below ; on this 
is seated the long tapering hair, which ends in a some- 
what recurved point or hook. The walls of the upper 
part of the hair are very strongly silicified, and are, con- 
sequently, easily ruptured. Lower down there is but little 
silica. When touched or rubbed by the hand, the pressure 
drives the hair downward ; at the same time the brittle 
hook penetrates the skin and breaks off. The downward 
pressure forces out from the broken hair a fluid of in- 
tensely acrid nature, which, on entering the wound made 
by the point, sets up more or less severe inflammation. 
This fluid is generally conjectured to be formic acid—a 
