642 



THE PATHOGENIC ALGAE. 



naked eye by bluish-green spots on sections of stems and 



rhizomes of the host -(Fig. 319). 



These originate as follows: the 

 species of Gunnera possess charac- 

 teristic mucilage-secreting organs, in 

 the form of fissures of the leaf-tip, 

 collaters on the leaves, and glands 

 on the stems. Merker^ found that 

 these glands originated endogenously 

 in the growing point between each 

 pair of leaves. The mature glands 

 are covered only by the epidermis, 

 and when activity commences the 

 uppermost cells of the gland-tip, 

 after swelling, become detached and 

 converted into slime (Fig. 320). 

 Ultimately the epidermis is ruptured 

 by the pressure of the slime, and 



the remaining glandular cells are in turn rapidly transformed 



into the same substance. 



Fig. 319. — Nostoc gunnerae. Longi- 

 tudinal section through the apex of a 

 stem of Gunnera vuxnicata. The punc- 

 tated spots indicate the position of 

 Nostoc-colonies. (v. Tubeuf deL) 



Fio. 320. — Nostoc gunnerae. Longitudinal sections through a gland in stem of 

 Ounnera ruacraphylla. e, Epidermis ; s, slime-canal ; no, colony of Nostoc. 

 (After Merker.) 



The Nostoc finds its way into the gland as soon as the 

 epidermis is broken, apparently attracted there by some 

 secretion. Merker found that the Nostoc filaments pass down 



^ Merker, " Ounnera macrophyUa," Inaugural Bissertation, 1888. 



