DISPERSAL AND INCREASE 257 



Crustaceous rock lichens are rarely sorediate, but they secure vegetative 

 propagation^ 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 obser- 

 vation 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^ 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 cir- 

 cumference 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 



' Beckmann 1907. '^ Bitter 1899. 



S. L. 17 



