The marshes of European countries, and also those of warmer lands, often contain bladderworts, or Utriculariæ (Fig. 23)--floating water-plants, without roots, and with horizontally spread, long-drawn-out, tendril-like shoots, in part thickly covered with whorls of delicate, needle-shaped leaves, in part bearing sparse leaves of quite peculiar structure. These are stalked, hollow bladders (Fig. 23 _A_, _FA_), with quite a narrow entrance at the apex, which is closed, as far as larger animals are concerned, by projecting bristle-like hairs (_B_). Small animals, such as water-fleas (_Daphnia_), species of _Cyclops_, and Ostracods, can swim in between the bristles, and they then come in contact with a valve which opens easily inwards (_B_, _kl_) and allows them to penetrate into the interior of the trap. Once inside they are captives, for the valve does not open outwards; therefore they soon die and decompose, and are then taken up by special absorptive cells (_B_, _C_, _sz_) and utilized as nourishment for the plants. In this way the Utriculariæ catch numerous little crustaceans and insect larvæ, which slip into their traps, presumably for concealment. [Illustration: FIG. 24. Pitcher of _Nepenthes villosa_, after Kerner. _St_, stalk of the leaf. _Spr_, its apex. _Fk_, the pitcher. _R_, the margin beset with incurved spines.] Another example is found in the marsh plants of the genus _Nepenthes_, some species of which live as climbers on the outskirts of tropical forests, climbing up the trees and letting their long, thin tendrils hang downwards, often over ponds and stagnant pools, where swarms of small flying insects abound. These plants have developed exceedingly remarkable contrivances for catching insects and using them as food (Fig. 24). The long stalks (_St_) of their leaves (_Spr_) are first bent downwards, then they suddenly turn sharply upwards, and the upturned portion is modified into a pitcher-like structure, in the bottom of which a fluid gathers, acid in taste, containing pepsin, and therefore a digestive fluid. Nitrogenous substances, such as flesh, dissolve in this fluid, and insects which fall into the pitcher from the rim are killed and dissolved. There are many species of _Nepenthes_, but not all of them possess the trap-structure in equal perfection, so that we are able, to some extent, to follow the course of its evolution, from a broad leaf-stalk, somewhat bent over at the edges, to the marvellous closed pitcher shown by _Nepenthes villosa_ (Fig. 24) of Borneo. In this species the pitchers attain a length of fifty centimetres, and are beautifully coloured, resembling in that respect, as well as in their form, the tobacco-pipe-like flowers of the tropical Aristolochiæ. When we come to discuss the origin of flowers, we shall see that the bright, conspicuous colour possesses a very considerable value in attracting insects; and in the case of the pitcher-plant, too, the gorgeous colour probably allures insects