Crustaceous rock lichens are rarely sorediate, but they secure vegetative propagation[917] by the dispersal of small portions of the thallus. The thalli most securely attached are cracked into small areolae which, by unequal growth, become very soon lop-sided, or, by intercalary increase, form little warts and excrescences on their surface. These irregularities of development give rise to more or less tension which induces a loosening of the thallus from the substratum. Weather changes act similarly and gradually the areolae are broken off. Loosening influence is also exercised by the developing fruits, the expanding growth of which pushes aside the neighbouring tissues. Wind or water then carries away the thalline particles which become new centres of growth if a suitable substratum is reached. _b._ DISPERSAL OF FOLIOSE LICHENS. It is a matter of common observation that, in foliose lichens where fruits are abundant, there are few or no soredia and _vice versa_. In either case propagation is ensured. In addition to these obvious methods of increase many lichens form isidia, outgrowths from the thallus which are easily detached. Bitter[918] considers for instance that the coralloid branchlets, which occur in compact tufts on the thallus of _Umbilicaria pustulata_, are of immense service as organs of propagation. Apothecia and pycnidia are rarely present in that species, and the plant thus falls back on vegetative production. Slender crisp thalline outgrowths, easily separable, occur also on the edges of lobes, as in species of _Peltigera_, _Platysma_, etc. Owing to the gelatinous character of lichen hyphae, the thallus quickly becomes soft with moisture and is then easily torn and distributed by wind, animals, etc. The action of lichens on rocks has been shown to be of a constantly disintegrating character, and the destruction of the supporting rock finally entails the scattering of the plant. This cause of dispersal is common to both crustaceous and foliose species. The older central parts of a lichen may thus have disappeared while the areolae on lobes of the circumference are still intact and in full vigour. As in crustaceous lichens the increase in the area of growth may take place by means of the lichen mycelium which, originating from the rhizinae in contact with the substratum, spreads as a hypothallus under the shelter of the lobes and far beyond them. When algae are encountered a new lobe begins to form. The process can be seen perhaps most favourably in lichens on decaying wood which harbours moisture and thus enables the wandering hyphae to retain life. _c._ DISPERSAL OF FRUTICOSE LICHENS. Many of these lichens are abundantly fruited; in others soralia are as constantly developed. Species of _Usnea_, _Alectoria_, _Ramalina_ and many _Cladoniae_ are mainly propagated by soredia. They are all peculiarly liable to be broken and portions of the thallus scattered by the combined action of wind and rain.