EXTERNAL CAUSES OF GROWTH AND FORMATION. II 321 



nutritive material. This active development of parenchyma is accompanied 

 by a reduction in the amount of sclerenchyma and collenchyma. It may be 

 assumed that these galls are induced by some substances excreted by the fungus 

 which operate in such a way as to stimulate growth in the host-plant just as 

 many poisons do. 



The alterations in form effected by other Fungi are even more peculiar than 

 these comparatively simple hypertrophies. Thus the whole appearance of cer- 

 tain species of Euphorbia is altered hy Uromyces pisi, a.nd Melampsorella cerastii 

 produces the well-known ' witch-brooms ' on the silver fir, altering dorsiventral 

 into radial shoots and perennial into annual leaves. Both of these forms of 

 Uredineae ripen their aecidiospores on the metamorphosed plant, and these 

 germinate on another host, producing in it no modification worth speaking of. 

 Whether that be due to an alteration in the fungus or to peculiarities in the 

 second host cannot at present be determined. Further examples of such modi- 

 fications of form are given by Peronospora violacea, which alters the stamens 

 of Knatitia arvensis into petals forming a ' double flower ', and by Ustilago anthe- 

 rarum, which stimulates growth in the otherwise dwarf stamens of the female 

 flower of Lychnis vespertina, and so apparently brings about the formation of a 

 hermaphrodite flower ; the anthers are, however, filled with the 

 reproductive organs of the fungus only and produce no pollen. 



The greatest modifications produced by Fungi are the 

 new formations, e. g. the spherical outgrowths of the alpine 

 rose which remind one of Cynips-ga\\s, but are really due to 

 the attacks of Exohasidium vaccinii, and especially the witch- 

 brooms induced by Taphrina laurenciana, that is to say, the 

 adventitious shoots with abnormal leaves which arise from the 

 foliage leaves oiPteris quadriaurita. [A very thorough study 

 of the anatomy of fungus-galls has been carried out by 



GUTTENBERG (1905).] 



The galls induced by insects are far more varied and, in 

 extreme cases, more complicated than fungus-galls. We may 

 note first of all cases where the organs of the plant attacked are 

 altered into others, and where the organs develop normally but 

 in unusual situations. Livia juncorum causes the altera- 

 tion of foliage-leaves in species of J uncus into scale-leaves, and 

 Chermes and Lonchaea lasiophthalma induce similar abnor- 

 malities in the spruce and in Cynodon dactylon (Fig. 94) re- 

 spectively. 



The transformation of floral-leaves into foliage-leaves 

 was induced by Peyritsch (1882) by inoculating species of Arahis with 

 aphides, and the same investigator was able to induce the formation of 

 vegetative-shoots in the flowers of Cruciferae and Valerianaceae by inoculation 

 with Phytoptus. 



Other types of gall arise by local hypertrophy in otherwise unaltered organs. 

 An example of this is seen in the bladder-galls of Viburnum lantana (Fig. 95), 

 where we meet with an enlargement only of the cells already formed ; usually, 

 however, active cell division takes place along with excessive superficial growth 

 or growth in thickness. Local surface-growth takes place on leaves in the common 

 bladder-galls ; local growth in thickness is illustrated by the very interesting 

 Cynips-ga.l\s, which deserve more detailed treatment. These are interesting in 

 the first place because they exhibit specific characters both as to form and size, 

 which become more prominent the larger they grow, and because they exhibit 

 peculiarities of internal structure which are unmistakably of the greatest impor- 

 tance, not to the plant, but to the insect causing the gall. We will, therefore, 

 study a characteristic example of a Cynips-gall both from the point of view of 



Fig. 94. Louchaea- 

 gall on Cynodon dac- 

 tylon. About half 

 natural size. 



