186 



BOTANT. 



343. They are all of rather small size, varying from a 

 millimetre or so to 20 or 30 cm. in length. For the greater 



Fig. 108.— a, a flat-growing ((oliaeeous) Lichen (Sticta puimonaria) ; B, 

 a stemmed (Iruticose) Liclien (Usnea barbata) ; a, a, fruit-disks (apotbe- 

 cia). Natural size. 



part the plant-body is flattish, and adherent to the sur- 

 face upon which it grows {A, Fig. 108), but some species 

 have more or less elongated branching stems {B). 



344. The plant-body of a lichen is composed of jointed, 

 branching, colorless filaments similar to those in the other 

 families of this order, but more or less compacted together 

 into a thallus or branching stem. They obtain their 

 nourishment from little green protophytes or phycophytes 

 to which the filaments attach themselves parasitically. 

 These little hosts, which live in the midst of the moist 

 tissues of the lichens, were until recently supposed to 

 be parts of the lichen itself, and were called gonidia, 



