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 Pinguecula, 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 Pinguecula 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 

 Pinguecula 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. 25 10 and 25 u ). 

 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 9 ). Its parts are a swollen disc 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 

 augmentatibn of the quantity of mucilage discharged, but does not cause secretion 

 of any acid digestive fluid. But as soon as a nitrogenous organic body is brought 

 into continuous contact with the glands, they are forthwith stimulated not only to 



