DEVELOPMENT OF PLANTS 253 



they are usually found, their growing periods are frequently very 

 short and their total annual growth may not exceed a few milli- 

 meters, 



(b) Reproduction of Lichens. — Reproduction of the lichens is 

 brought about by means of fragments that are readily detached 

 when the lichens are dry and brittle, or by means of soredia. 

 These latter bodies consist of a few algal cells intertwined with 

 hyphae, in fact, a miniature lichen (Fig. 177, C). In some spe- 

 cies the soredia form a rather powdery or granular coating on 

 the upper surface of the lichen, and in other cases they are devel- 

 oped within the lichen. These bodies are easily scattered by the 

 wind when the lichens are dry and under favorable conditions 

 grow into new lichens. Reproduction is also affected by means 

 of spores that are formed in asci since the species of fungi appear- 

 ing in the lichens are largely ascomycetes. These reproductive 

 parts appear as small open or closed ascocarps which develop a 

 hymenial layer of asci and paraphyses, as already noted in the 

 ascomycetes (Fig. 178). The ascocarps assume a great variety 

 of forms, as seen in Fig. 176, as. In two genera the spores are 

 borne on basidia and it is evident that certain species from a 

 widely different group of fungi, the basidiomycetes, have acquired 

 the habit of association with the algae. The origin of the asco- 

 carp from a female gametangium has been observed in a few 

 cases, and it is interesting to note that the male and female organs 

 bear a more striking resemblance to those found in the red algae 

 than has been observed among the ascomycetes. The small 

 spores frequently developed in ascocarp-like bodies, as shown in 

 Fig. 176, C, are considered as the male gametes. 



The ascospores germinate readily, producing hyphae which, 

 however, soon perish unless they chance to meet an alga with 

 which they can live. It should be noted that these fungi appear- 

 ing in lichens cannot associate indiscriminately with any species 

 of algae. Each species of fungus is adapted to one or more spe- 

 cies of algae and is unable to live with any other form. As may 

 be imagined, the real nature of the lichen was for a long time 

 misunderstood, the algae even being looked upon as spore bodies. 

 The bitter dispute over the question was finally settled when a 



