THE FACTORS OF DISTRIBUTION 63 



spines, attach themselves as burs to passing animals and 

 may thus be transported to a distance before rooting. 



The Frog-bit (Hydrocharis Morsus-ranca L.) lives 

 mostly in stagnant water, but winter-buds detach them- 

 selves from its slender submerged branches, and may 

 rise to the surface and develop into new plants at some 

 small distance. The Water-soldier (Stratiotes A hides L.), 

 the Bladderworts (Utricularia], and the Pond-weeds 

 (Potamogeton) multiply by similar methods. The sub- 

 terranean migration of tubers, bulbs, and rhizomes, as, 

 for example, in the Lily-of-the-valley, is in general very 

 slow, although sometimes facilitated by the contractility 

 of horizontal roots. The Butter-bur (Petasites hybridus 

 Gart.), however, will form rhizomatous shoots i metres 

 in length in a single season. The rapid spread of Straw- 

 berry plants over the surface of the ground by means of 

 their runners is another familiar example of vegetative 

 migration. 



The adaptations for dispersal in spores, fruits, and 

 seeds are. perhaps, for our present purpose, best con- 

 sidered under four groups, dependent on the agency of 

 dispersal rather than on the structural nature of the 

 dispersal mechanism, since in each group we find a very 

 great variety of structures modified to perform similar 

 functions. These four main groups have been termed 

 bolochores or sling-fruits ; hydrochores, plants dispersed 

 by water; anemochores, those dispersed by wind; and 

 zoochores, those dispersed by animal agency. 



The extreme lightness of the minute spores of many 

 Fungi, Mosses, Club-mosses, and Ferns; of the seeds of 

 many Orchids, especially those that are epiphytic; of 

 many parasites, saprophytes, and other plants, will facili- 

 tate their dispersal, whether by propulsive mechanism, 

 wind, or water. The seeds of Goo dyer a repens Ait., 

 for example, weigh two-mil lion ths of a gram. 



BOLOCHORES. The bursting of puff-balls as they dry, 

 and the scattering of moss-spores between the teeth of 

 the peris tome as the capsule sways in the wind, are 

 analogous, among cryptogams, to the bursting of the 

 drying pods of Broom and Gorse, and to the " censer- 

 action " by which the wind, swaying the dried stalks, 

 shakes seeds between the teeth of the fruit of a Pink, 

 or through the pores of a Poppy-head. The mere 

 drying-up of the tissue of the carpels, causing them to 



