EFFECTS OF PARASITIC FUNGI ON TISSUES OF HOST. 43 



the witches' brooms due to Aecidium elatinum, the pith appears 

 enlarged as the result of increase of the medullary parenchyma. 

 In diseased leaves, palisade parenchyma- can frequently no longer 

 be distinguished from spongy, and only irregular polygonal cells 

 are formed. As examples may be given needles of fir with 

 Aecidium abietinum, and leaves with galls due to Exoasceae. 

 Finally, there may be a marked increase of wood-pareijchyma, 

 both of medullary rays and the wood proper; this is especially 

 well marked in Juniperus communis affected by Gymnosporangium 

 juniperinum} where in consequence of an enormous increase of 

 the parenchyma of rind and medullary rays, the tracheidal 

 regions become separated by broad wedge-shaped rays, and at the 

 same time they are peripherally intersected by bands of paren- 

 chymatous tissue resulting from increased development of the 

 wood-parenchyma (Fig. 220, etc.). 



The Sclerenchyma is generally suppressed where hypertrophy 

 occurs. Examples mentioned by Wakker are stems of cowberry 

 with Exohasidium, of hawthorn with Gym,nosporangium, of 

 Sanguisorhia with Xenodochus, and alder catkin-scales with 

 Homascus. On the other hand, sclerenchyma is developed in 

 stems of Cirsium as a result of Puccinia suMveolens, whereas 

 normally it is absent. 



The secondary vessels of the wood frequently remain irregular, 

 and with imperfectly absorbed partition-walls. According to 

 Wakker, this is the case in Vaccinium with Exoha-sidium, 

 Crataegus with Boestelia, and Rhamnus with Aecidium,. 



Suppression of interfascicular cambium was observed by 

 Wakker in buckthorn and nettle with their respective Aecidium 

 parasites. Prolonged activity of the same tissue he found in 

 Sisymbrium with Cystopus. 



Arrest of lignification was found by Wakker in medullary rays 

 of Crataegus with Roestelia, and in deformed scales of alder 

 catkins affected by Exoascus. 



We have already considered increased growth in length and 

 thickness in connection with hypertrophy. It need only be 

 added that increased thickness of woody plants may be due to 

 increase of the rind, the bast, the pith, or medullary rays, and not 



'■ P. Womle. " Anatomische Untersuchung d. diirch Gymnosporangium-Arten 

 hervorgerufenen Missbildungen. " Inaiig. Diss., Munchen, 1894; also, Forstlich- 

 ■naturwiss. ZeiUchrift, 1894. 



