PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY. 141 
springs, banks of brooks, moorlands, and black peat-bogs. In the equatorial zone 
they have retired into the cool regions of the higher mountains. The mountain 
ranges of Mexico are particularly rich in species of Pinguicula, but all the forms 
existing there occupy a circumscribed area. Southern and western Europe also 
harbour a few native species whose area of distribution is surprisingly limited. 
Thu species occurring in the arctic and sub-arctic zones are, on the contrary, exceed- 
ingly widely distributed. One species has been found in antarctic regions at the 
Straits of Magellan. 
The species best known and most available for study is Pinguicula vulgaris. 
The area of its distribution extends over the whole of the arctic and sub-arctic 
regions, over the part of North America which lies to the north of the Mackenzie 
River, over Labrador, Greenland, Iceland, and Lapland, throughout Siberia down 
to the Baikal Mountains, and through Europe to the Balkans, Southern Alps, and 
Pyrenees. This graceful plant is represented on its natural scale and growing on 
a bog in the annexed Plate II. entitled, “Insectivorous Plants: Sun-dew and Butter- 
wort”. It has bilabiate flowers of a violet-blue colour, with palates covered with 
velvety-white hairs, and with a sharp spur at the back. The flowers are borne 
singly on slender stalks which rear themselves in an elegant curve from the centre 
of a rosette of leaves that rests upon the ground. The leaves of the rosette in 
Pinguicula vulgaris, as in all other species of Butterwort, are oblong-ovate or 
ligulate and of a yellowish-green colour, and rest their under-surfaces upon the wet 
ground, whilst their upper faces are exposed to the sky and rain. Owing to the 
lateral margins being somewhat upturned, each leaf is converted into a broad flat- 
bottomed trough (cf. the section taken right across a leaf in fig. 251° and 251). 
The trough is covered with a colourless sticky mucilage which is secreted by glands 
distributed in large numbers over the entire upper surface of the leaf. 
The glands are of two kinds. One variety is distinguishable by the naked eye 
as consisting of a stalked head, and looks under the microscope like a tiny mush- 
room (see fig. 25°). Its parts are a swollen dise composed of from eight to sixteen 
cells grouped radially, and a stalk, consisting of an erect tubular cell supporting this 
disc. A gland of the other sort is made up of eight cells grouped in the form of 
a wart or knob supported by a very short stalk-cell, and only slightly raised above 
the surface of the leaf. For the rest, ordinary flat epidermal cells make up the 
epidermis, with here and there interspersed the guard-cells of stomata. 
It has been calculated that there are 25,000 mucilage-secreting glands on a 
square centimeter of a butterwort leaf, and that a rosette composed of from six to 
nine leaves bears about half a million of them. Momentary contact, whether due to 
rapid brushing by a solid body or to the incidence of drops of rain, causes no kind 
of movement in them. The long-continued pressure of grains of sand or of solid 
insoluble bodies in general stimulates the glandular cells to an inconsiderable 
augmentation of the quantity of mucilage discharged, but does not cause secretion 
of any acid digestive fluid. But as soon as a nitrogenous organie body is brought 
into continuous contact with the glands, they are forthwith stimulated not only to 
