181 



of Juniperus coimnyiis attacked by.. Gymnosp oranglum junlperinum 

 (the lorm living in the needles) A — : 



The same arrestment of tissue differentiation is found 

 in Zoocecidiae. Figure 75 shows a cross-section of h leaf of 

 Crataegus oxyacantha after colonization by Aphis oxya canthae. 

 a leaf-louse. On the infected swollen lolanes, usually strik- 

 ingly reddened, the mesophyll is increased several cell layers, 

 any distinct formation in the upper and lov/er layers is entire- 

 ly lacking, and the epidermal cells are approsiinately as large 

 as those of the neso-phyllt Its cell walls are often pretty 

 thick and' apparently very rich in water. 



In all cases the hyperplastic cell increase is limited 

 to the mesophyll while the epidermis participates only in so 

 far as its cells are often considerably enlarged, cross-divi- 

 sions taking place only exceptionally* That th.e ground tissue 

 tends to a greater proliferation than does the epidermis has 

 been repeatedly confirmed above and will be- reaffirmed later. 



While, in most cases, in the proliferation of the meso- 

 phyll as in pure hypertrophic changes (see above) , the chlo- 

 rophyll content da^enerates completely or in great part, there 

 (201) are ^ a few exceptions among galls in which the chloroplasts 

 are retained. Such exceptions are shown by the so-called 

 pustules of Pirus, Sorbus and others* Through the action of' 

 gall-mites mesophyll is forced out into elliptical swellings, 

 the single cells are g reatly elongated and many times divided:,, 

 their content retaining its normal green color* In the placets 

 infected the le&f tissue consists of porous green, confervoid 

 rovers of gsSifcs, The excrescences of the assirailatory tissue in 



1. W'ornle. Anat. Unters. der durch GsTcnosporangium-Arten 

 hervorgeruf enen Missbildungen. Porstl.-Iaturwiss. Zeitschr., 

 ,1894, Bd. Ill, p. 68. Of the other literature, compare es- 

 pecially, TiJoronin, Exobasiclium Taccinii. Verh. naturforsch. 

 Ges, Freiburg i. Br. 1867, Bd. IV, p. 397. Wakker, Unters. 

 {ib, d. Einfl, parasitcher Pilze auf ihre N'^pfl, Versuoh ein- 

 er,.pathol. Anat. d. Pfl. , Pringsheim*s Jahrb. f, wiss. Bot. , 

 1802, Bd. ZXIT, p. ^499. Further Fentzling, Morph. u. anat, 

 Untersuch. der Teranderungen, welche bei einigen Pfl. durch 

 Rostpilze hervorgeruf en werden. Diss, Freiburg i. Br. 1892, 

 Peglion, Studi anatom di alcune ipertrofe indotte dal Gystopus 

 oanidus in alcuni organi die Haphanus Raphanistrum. Riv. Pat. 

 ve^, 1892, Vol. I, p. 265. Smith, W. G., Un-bers.d. Morph. u. 

 Anat, der durch Exoasceen verursachten Spross- u. Blattdeforma- 

 tionen. Gorstl,- Haturw. Zeitschr., 1894, Bd. Ill, p. 420, 

 Therein a niimber of graphic illustrations, Molliard, ^feoidies 

 florales. Ann. Sc, 'Hat. Bot., 1895, S^e ser,, T. I, p. 67. 

 Strohmeyer, 0. » Anatom, Untersuch. der durch Ustilagineen her- 

 vorgeruf, Missbildungen. pissertation Erlangen, 1896, ^feneau 

 Be lamarliere, S. 1, mycocecidies du Roestelia. Rev, gen. 



ie Bot,, 1898, T. X. p. 225. 



2. In contrast to the many cases already described of chlo- 

 rophyll degeneration and its non-development, the present caseS 

 are of especial interest since there is no lack of food sub- 

 stances here for the colorless cells, or those poor in pigment. 

 The cytoplasm is rich in protein and the cells are usually 

 abundantly provided with starch meal. The causes determining 

 the utilization of the food substances for the formation of 

 chlorophyll are unknown. 



