TYPES OF FUNGAL GALLS. 519 



(cf. vol. i. p. 508). In fermentation the chemical composition of the fluid is 

 altered, its chemical compounds are shaken, decomposed, and split up and new 

 compounds are formed by the action of the living Yeast cells. The same thing 

 happens here in the interior of the living plant in its turgid, meristematic tissue 

 that is to say, in a group of protoplasts which still have the power of growing 

 at the expense of materials supplied them, of increasing in size, and of multiplying 

 by division. But these cells no longer behave as in the absence of the parasite 

 they would have done. Profoundly modified under the influence of the parasite, 

 but yet not killed, these cells, by their continued division, form tissues and organs 

 of new and unusual form; in other words, that part of the host which is invaded 

 but not killed by the parasite will continue to grow and increase in size, and in 

 consequence of the change which its protoplasm has experienced will assume a 

 different outward form. 



These altered tissue-bodies produced by parasitic Fungi are called gall-structures. 

 They are usually characterized by an excessive growth known as hypertrophy, as 

 well as by their altered shape. The hypertrophy is without doubt caused by a 

 stimulus proceeding from the parasite. We may conclude that the significance 

 of the increased growth lies in the abundant supply of nourishment thus placed 

 at the disposal of the parasite, since the large quantity of food-material brought 

 for the excessive development of the hypertrophied growth connotes a large supply 

 for consumption by the parasite. In many cases, however, the hypertrophied tissue 

 merely forms a wall protecting the host against the further depredations of the 

 intruder. It then contains no nourishment for the use of the parasite, being built 

 up chiefly of corky cells, which the latter cannot consume or destroy. Such a tissue 

 might be compared to the so-called callus which grows up in plants in parts de- 

 prived of epidermis after an injury, or in other wounds, and gradually covers them 

 over with a protective layer. 



The formation of the gall is often restricted to only a small portion of the 

 afflicted plant; in other cases whole leaves and branches, and sometimes even ex- 

 tensive shoots, become modified in shape. To get a general idea of the four types 

 of hypertrophied growths it will be best to take them one after the other in the 

 order mentioned, commencing with the simplest. 



The simplest of these galls consist of a few degenerate and metamorphosed cells 

 in the centre of an extensive and unaltered tissue. They are produced chiefly by 

 parasites of the genera Rozella, Synchytrium, Exobasidium, and Gymnosporan- 

 gium. Rozella septigena, one of the Chytridiese, develops swarm-spores which 

 attack the various species of the fungal genus Saprolegnia. They settle on the 

 tubular branches of the Saprolegnia at a place where it was just about to divide 

 and to produce swarm-spores of its own. In consequence of the invasion of the 

 parasite this does not take place, but the tubular cells which would have formed a 

 Saprolegnia-sporsmgium divide instead into short barrel-shaped cells, each of which 

 becomes a sporangium of Rozella septigena. In addition to this the infected cells 

 develop lateral outpushings which swell up spherically, and each contains a resting- 



