482 Pear son . — Anatomy of the Seedling of 
in a continuous sheath of cork. As in the stalk, no phellogen 
is formed in the pericycle. 
As has been stated above, most of the apogeotropic roots 
examined, particularly the older ones, contained colonies of 
Anabaena. The Alga is found in the intercellular spaces 
between the cells of a ‘ palisade-layer * which occurs midway 
between the periphery of the root and the stele (Fig. 11, i. c .). 
The ‘blue-green’ ring thus formed is visible to the naked 
eye in a transverse section. The Alga flourishes most 
abundantly in the tissues of the ‘ head ’ ; but in the older 
roots it is present in a similarly well-defined ring for a con- 
siderable distance down the stalk, becoming less abundant 
lower down, and, in all cases yet examined, disappearing 
entirely above the junction of the stalk with the main root. 
The c palisade-layer ’ is not seen in roots which contain no 
Anabaena , and it would seem that the characteristic appear- 
ance of the layer is due very largely to the lateral crushing, 
and consequent radial extension of its cells, by the pressure 
upon them of the developing colonies of Anabaena (Figs. 12 
and 13). In roots in which the Alga is beginning to develop 
it occurs in small patches between the cells ; as it becomes 
more abundant the cells are forced into the shape represented 
in Fig. 12 , and finally the lumina become quite obliterated, 
so that neighbouring colonies are separated by radial par- 
titions which can be recognized as collapsed cells only by 
treatment with re-agents. In this respect the method of 
occurrence of the Algal colonies in the apogeotropic root 
of Bowenia differs from that described by Reinke for Cycas , 
in which case a true palisade-layer of very definite form is 
present 1 . With the exception of a few isolated groups, the 
Alga is quite confined to this mid-cortical zone, which no- 
where approaches nearer the surface of the root, and in many 
cases forms an uninterrupted layer parallel with the surface 
of the root. 
The Alga is a typical Anabaena , showing characteristic 
heterocysts. 
1 Reinke, loc. cit., Bot. Ztg., 1879, see Figs. 2 and 3. 
