AECIDIUM-FORMS. 



407 



iu the growth of the wood disturbs the elements, so that 

 they are irregularly developed and more or less twisted.^ 



A mycelium inhabits the tissues of abnormal twigs and 

 cankered swellings. It grows in the . intercellular spaces of 

 the rind, between the bast cells and outer parts of the wood, 

 and derives nutriment by means of haustoria ; these either bore 

 through the cell-walls, or only press closely against them so 

 as to cause depressions. 



Spore-formation takes place on the needles of the witches' 

 brooms. The pycnidia (spermogonia) are produced on the 

 upper side beneath the cuticle 

 and emerge through it as little 

 yellow points. The conidia (sper- 

 matia) are tiny globose colourless 

 bodies. The aecidia come later 

 during June and July in irregular 

 rows on the under side of the 

 leaf Their peridia break out as 

 low dome-like structures, the 

 apices of which rupture irregu- 

 larly to allow escape of aecidio- 

 spores. In spite of numerous 

 infections, De Bary was unable 

 to observe the penetration of a 

 germ-tube into needles or twigs 

 of silver fir. Weise believes that 

 infection of the fir takes place 

 on twigs which have just emerged 

 from the bud. 



As a preventive measure, all 

 witches' brooms should be cut off' 

 before spore-formation begins, and 



stems with canker- wounds should be removed during forest- thinning. 

 For further details the monograph of Heck may be consulted. 



Aecidinm strobilinum (Alb. et Schw.)^ (Britain). Spruce- 



FlG. 2i3.—Aecidi-um strobilinum on a Spruce- 

 cone, (v. Tubeuf phot.) 



^Note. — Further details of the anatomical changes induced in the tissues of 

 these witches' brooms may be obtained in the German edition of this work 

 (pp. 420-421), or in the original thesis by Hartniann, [Anatom. Vcrgleichung d. 

 Hexenbesen der Weisstanne. Inaugural Dissertation, 1892.) (Edit.) 



^Reess, Sostpihformen d. Ooniferen., 1869. Oerstedt, Naturh. for Videiislc. 

 Medd., 1863, i. 



