SAPROPHYTISM AND SYMBIOSIS 785 



comparable modifications, due to similar causes, occur in the leaves of Anemone 

 quinquefolia and Hepatica acutiloba. Parasitic seed plants less commonly cause 

 galls, though Conopholis sometimes occasions conspicuous enlargements on oak 

 roots, and the mistletoe frequently occasions stem swellings and variations in the 

 form of the branches. 



The cause of gall formation. Although galls obviously are caused by 

 foreign organisms, the exact nature of the stimuli involved is unknown, 

 and can scarcely be determined precisely until galls are produced arti- 

 ficially under controlled conditions. Some investigators regard chemi- 

 cal stimulation as the ruling factor, while others think that mechanical 

 irritation is more important; still others appeal to the possibility of 

 osmotic changes, a view suggesting analogies with tuberization. Some 

 investigators think that a brief contact with a foreign structure (as with 

 the ovipositor of an insect) is sufficient for gall formation, while others 

 think that long-continued stimulation (as by insect larvae) is necessary. 

 The most remarkable galls are those in which new structures appear, as 

 in the galls of the rose, the grape, and the oak, with their prickles or hairs. 

 In some cases, as in various so-called domatia, gall-like structures 

 appear to develop without stimulation by foreign organisms. For 

 example, the myrmecophytes, Hydnophytum and Myrmecodia, have 

 large tubers permeated with air chambers and passages that are in- 

 habited by ants, but it has been shown that the chambered tubers de- 

 velop independently of ant stimulation. Recently it has been dis- 

 covered that some witches' brooms (as in the spruce) can be propagated 

 by seed, many of the progeny from such shoots being dwarf and bushy. 

 There has been advanced the somewhat dubious theory that structures 

 of this sort once were due to the stimulation of the foreign organism, 

 but that now gall formation has become an inherent feature of the 

 plant. 



The advantages of galls. Unlike most plant structures, galls are 

 obviously disadvantageous to the plants of which they form a part. The 

 energy and material used in their construction, the food which they 

 accumulate and which is utilized by the foreign organism, together with 

 many activities of the parasites are features of positive detriment. Thus 

 galls furnish one of the best illustrations of the fallacy of the theory of 

 adaptation. In a few cases galls are believed to be advantageous to the 

 host as well as to the parasite, notably in the root tubercles of the legumes 

 (p. 790); if the fungus theory of tuberization is confirmed, there be- 

 comes evident another striking case of gain through gall formation. 



