NOSTOCACE^:. 261 



viz., Azolla caroliniana. This Azolla winters in plant-houses, 

 and we are therefore in a position to obtain material at any time 

 for the study of the Nostocaceae. The Nostocaceae are, in 

 general, specially disposed towards symbiosis (i.e., interrelations 

 with other plants), and we find them in very various plants, 

 especially, however, as constituents of the body of Lichens. The 

 Anabana Azollce, living in the Azolla, is found in very definite 

 parts of the plant in question. The leaves of Azolla are each 

 two-lobed. The upper lobe is fleshy, and floats on the water ; 

 the under is membranous, and immersed. The upper lobe shows 

 in the interior a broad hollow, into which a narrow opening, 

 found on the inner surface of the leaf, leads. This cavity is filled 

 with Anabc&na, and from the walls of the hollow- branched hairs 

 also grow between the coils of this Anabcena. In order to obtain 

 the Anab&na for examination, we pull the upper lobes of some 

 leaves to pieces with the needles, lay on a cover-glass, press upon 

 this a little, and are now pretty sure to find the Anab&na threads. 

 This much is certain, that no specimen of the Azolla is devoid of 

 them. We examine the threads with our high- 

 est available power (Fig. 99). They consist 

 of a row of very delicate-walled barrel-shaped 

 cells, which from time to time are interrupted 

 by a larger, ellipsoid or globular cell, the hetero- 

 cyst, in which, at the places of union with the 

 adjoining cells, small highly-refractive projec- 

 tions are found. The threads are serpentine, 

 coiled here and there, without any recognis- 

 able gelatinous coat. The contents of the 

 vegetative cells are coloured verdigris-green, 

 of the heterocysts olive-green ; small, darker- 

 looking granules are distinguishable in these 

 contents; a nucleus is not recognisable. In- 



dividual cells are usually found in division by cessive stages in the 

 T i_ A? /-r-i- rr* 7\ e n i division of vegetative 



median constriction (Fig. 99, a to d), followed cells; &, a limiting cell, 

 by the formation of a partition wall. If a twig or heterocyst(x 540). 

 of Azolla is taken between the fingers, and surface-sections 

 taken from it, not infrequently the Anabcena can be seen under 

 the microscope in its natural position inside a leaf-cavity. The 

 leaf-cavity must, however, have been cut in the proper direction, 

 and then we can also see the jointed hairs which accompany 

 the Anabcena. The addition of iodine solution colours the contents 



