VIII] 



URKDINALKS 



213 



Moreau describes only two, but I [arper and 1 [olden found a larger number, 

 which became crowded together and more or less fused during the later 

 stages of the first and also during the second division. 



In Gymnosporangium clavariaeforme ( fig. 190) the first divisii >n is initiated, 

 as in Coleosporium, by a synaptic phase, after which a spireme is formed and 

 breaks up into chromosomes. 



These pass on to the spindle M /.' \ ■'■'.. ,y 



but soon lose their individuality \ 

 and travel in irregular masses 

 to the poles. 



The development of the 

 spindle has also been traced in 

 this species ; its formation is 

 extra-nuclear and it lies free in 

 the cytoplasm before coming 

 into relation with the dividing 

 nucleus. This type is fairly 

 common among animals but is 

 of exceptional occurrence in 

 plants. 



Nuclear Association. The 

 cytology of the aecidium was 

 first described in detail in 1904 

 In' Blackman, for Phragmidium 

 vio/aceum, a species occurring on 

 the bramble. The aecidium here is of the caeoma type, consisting of a group 

 of fertile cells of indefinite extent and usually bounded at the periphery by 

 a number of thin-walled paraphyses. 



Its formation begins by the massing of hyphae below the epidermis of 



the leaf where they form a series of uninucleate cells two or three layers 



thick. The cells next the epidermis increase in size and each divides by a 



transverse wall parallel to the surface of the leaf, separating an upper sterile 



cell from the fertile cell below. 



The sterile cells remain cubical 



and ultimately disintegrate; the 



fertile cells elongate to form a 



more or less regular layer and 



paired nuclei appear in them, 



first at the centre and later 



towards the periphery of the 



group (fig. 191 ). 



„. , , ... Fig. 191. Phragmidium vielaceum Went.; caeoma, 



The second nucleus in the ,, 40: , lMrl Blackman. 



Fig. 190. Gymnosporangium clavariaeforme Rees 

 first division in basidium, x [460; after Blackman. 



