377 
Life-history of Anabaena Cycadeae. 
small spherical protoplasts are formed inside the original heterocyst walls 
(Fig. 25). These under certain conditions are liberated by the opening 
of one of the pores in the wall (Figs. 24, 25)* The gonidia, when set free, 
have pale green contents, surrounded by an exceedingly delicate membrane, 
which soon becomes more definite, and the outward investment appears, 
also a very distinct granule lying near the centre of the cell, which no 
doubt represents the visible portion of the central body. Each of these 
small bodies is capable of resting for a time or germinating to form a small 
but typical Anabaena filament (Fig. 26), by division comparable to the 
process in a vegetative cell. Increase in size occurs both before and after 
division. 
In view of the fact that the old material contained many spores, it is 
noteworthy that their contents on liberation are capable of differentiating 
directly into a heterocyst, which can then produce gonidia. 
The formation of gonidia in Anabaena Cycadeae , besides supporting 
the proposition that heterocysts are the abortive relics of a method of 
reproduction once prevalent among the Cyanophyceae, also offers some 
solution to the problem : How does the Anabaena get into the Cycas 
tubercle ? Since the Alga is not present in very young tubercles, it must at 
some time find an entrance, and must also be living in the medium around 
the roots, the soil. Means of entry appear to be afforded by the lenticels, 
which are present on the tubercles, through which the organism could 
readily enter as a small gonidium, and it is difficult to conceive how it could 
get through such a small opening in any other form. Anabaena Cycadeae 
is undoubtedly set free in the soil by the decay of old tubercles, in the form 
of spores and heterocysts, which under certain conditions give rise to 
gonidia, capable of forcing their way through the lenticels into the Cycas 
tubercle, where they encounter conditions favourable to their growth, and 
consequently there they flourish, producing vegetative cells, spores, and 
heterocysts. 
Having entered the nodule, they confine their growth to a particular 
zone about four layers deep, and here, after their entrance and growth, the 
intercellular space, known as the algal zone, arises. It is very probable 
that conditions of aeration and illumination play an important part in the 
selection of this portion of the nodule, because in culture experiments where 
sections were employed the Anabaena spread to other regions besides the 
zone ; here, however, conditions of illumination and aeration would be 
practically uniform throughout the section. Thq Anabaena , having chosen 
their home in the nodule, are very soon surrounded by the nitrogen-fixing 
bacteria, one of which, Azotobacter , entered the lenticels, like the Anabaena, 
whilst the other, Pseudomonas radicicola , was already established, and was 
indeed the primary cause of the tubercle formation. There is thus in the 
algal zone a wonderful symbiotic community consisting of an organism 
