302 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



brooms," strikingly modified branch systems, which are caused by parasitic 

 fungi. A very interesting witches' broom is produced by the fungus Taphrina 



laurentia upon the fern Pieris quadriaurita, as 

 is shown in Fig. 149. These curious outgrowths 

 are always formed on the upper side of the leaf, 

 and they grow upward in such manner as to 

 suggest that another leafy plant has established 

 •itself upon the fern. They resemble similar 

 lateral outgrowths found on the leaves of fossil 

 ferns. 



From the point of view of plant phylogeny, n 

 it is sometimes possible to throw light on genetic 

 relationships by the study of pathological phe- 

 nomena that may include the formation of atavis- 

 tic structures. These latter may be apparently 

 quite new for the plant in question , but may be ac- 

 tually like structures that were usual in its remote 

 ancestors. 1 Thus, the compound flower-heads 

 of Crepis biennis, when infected with the mite 

 Eriophyes, are very different from the uninfected 

 heads, and the modification appears to be an 

 atavistic one, reverting to an ancestral type 

 (see Fig. 150). Also, the dioecious plant Meland- 

 ryum album bears perfect ("bisexual") flowers 

 when infected with the parasitic fungus Ustilago 

 anther arum (see Fig. 151). 

 As has been stated (page 251), some tissues in ordinary plants are subjected 

 to traction, while others are subjected to pressure. An artificial pull may be 

 applied to a plant, to determine the effect of traction upon growth. Hegler's 2 



Fig. 150. — Flower-heads 

 Crepis biennis; two unmodified, 

 and two modified by the presence 

 of the mite Eriophyes. 



Fig. 151. — Flowers of Melandryum album, in vertical section. The normal staminate and 

 pistillate flowers are shown at left and right, respectively, and the middle diagram represents a 

 perfect flower (with both stamens and pistils), this modification being produced by the pres- 

 ence of the fungus Ustilago antherarum. 



1 Potonie, H., Grundlinien der Pflanzen-Morphologie im Licht der Paleontologie. Jena, 1912. 



- Hegler, Robert, Ueber den Einfluss des mechanischen Zugs auf das Wachsthum der Pflanze. Cohn's 

 Beitrage zur Biol. d. Pflanzen. 6: 383-432. 1893. [Newcombe, Frederick C, The regulatory formation of 

 mechanical tissue. Bot. gaz. 20: 441-448. 1895- Pieters, Adrian J., The influence of fruit-bearing on 

 the development of mechanical tissue in some fruit-trees. Ann. bot. 10: 511-529. 1896.] 

 " This paragraph appears for the first time in the 7th Russian Edition. — Ed. 



