Soralia of definite form are of rather rare occurrence in crustaceous lichens, with the exception of the Pertusariaceae, where they are frequent, and some species of _Lecanora_ and _Placodium_. They are known in only two hypophloeodal (subcortical) lichens, _Arthonia pruinosa_ and _Xylographa spilomatica_. Among squamulose thalli they are typical of some _Cladoniae_, and also of _Lecidea_ (_Psora_) _ostreata_, where they are produced on the upper surface towards the apex of the squamule. [Illustration: Fig. 83. _Parmelia physodes_ Ach. Thallus growing horizontally; soredia on the ends of the lobes (S. H., _Photo._).] _b._ POSITION OF SORALIFEROUS LOBES. According to observations made by Bitter[492], the occurrence of soralia on one lobe or another may depend to a considerable extent on the orientation of the thallus. He cites the variability in habit of the familiar lichen, _Parmelia physodes_ and its various forms, which grow on trees or on soil. In the horizontal thalli there is much less tendency to soredial formation, and the soredia that arise are generally confined to branching lobes on the older parts of the thallus. That type of growth is in marked contrast with the thallus obliged to take a vertical direction as on a tree. In such a case the lobes, growing downward from the point of origin, form soralia at their tips at an early stage (Fig. 84). The lateral lobes, and especially those that lie close to the substratum, are the next to become soraliate. Similar observations have been made on the soraliferous lobes of _Cetraria pinastri_. The cause is probably due to the greater excess of moisture draining downwards to the lower parts of the thallus. The lobes that bear the soralia are generally narrower than the others and are very frequently raised from contact with the substratum. They tend to grow out from the thallus in an upright direction and then to turn backwards at the tip, so that the opening of the soralium is directed downwards. Bitter says that the cause of this change in direction is not clear, though possibly on teleological reasoning it is of advantage that the opening of the soralium should be protected from direct rainfall. The opening lies midway between the upper and lower cortex, and the upper tissue in these capitate soralia continues to grow and to form an arched helmet or hood-covering which serves further to protect the soralium. [Illustration: Fig. 84. _Parmelia physodes_ Ach. Thallus growing vertically; soredia chiefly on the lobes directed downwards, reduced (M. P., _Photo._).] Similar soralia are characteristic of _Physcia hispida_ (_Ph. stellaris_ subsp. _tenella_), the apical helmet being a specially pronounced feature of that species, though, as Lesdain[493] has pointed out, the hooded structures are primarily the work of insects. In vertical substrata they occur on the lower lobes of the plant.