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PROFESSOR A. ANSTRUTHER LAWSON ON 
due to the difference in habitat. Localities were found where the plants grew in 
abundance. It should be pointed out that while one may occasionally find Tmesipteris 
and Psilotum growing together, that is the exception, not the rule. The rule is 
that the Tmesipteris is found deep in the gullies, or in wet, sheltered, shady places, 
protected from the winds by cliffs or overhanging rocks, and by vegetation where 
Dicksonia or Alsophila or Todea are to be found. In New South Wales it seems to 
reach its maximum development on the trunks of DicJcsonia. Psilotum , on the other 
hand, flourishes best in just the opposite kind of habitat. One may find it in great 
abundance in the immediate environs of Sydney growing out of the crevices in the 
sandstone cliffs exposed to the sun and winds. It would be difficult to imagine two 
habitats so utterly different. Although the dry xerophilous conditions are character- 
istic for Psilotum , I have found it often growing luxuriantly in a saturated 
atmosphere within the spray of waterfalls. It is under these latter conditions that 
one is more likely to find the prothalli. I have occasionally found prothalli in the 
soil contained in the deep crevices of the sandstone rocks, by chiselling away the 
rocks and gathering the soil in which young plants were growing. It is easier, how- 
ever, to obtain the prothalli from soil near water, where the older plants have been 
established for years, and have been continually shedding their spores in the wet 
sandy soil. The moisture is not only necessary for the germinating of the spore, but 
it seems also to bring about a condition suitable to the endophytic fungus which 
infects the cells of the young prothalli. From such wet sandy soil patches were selected 
containing young sporophytes, and this material was later carefully worked through 
with a lens, by the same method as that used in finding the prothalli of Tmesipteris. 
It proved to be a much easier and more certain method than that first tried of chisel- 
ling the rock away and examining the soil in the crevices. In the free wet soil I 
was successful in finding several small pockets of prothalli, and these were enough 
to show all the essential features, vegetative and reproductive. 
As in Tmesipteris , Psilotum has a subterranean prothallus. None of the speci- 
mens obtained were less than half an inch below the surface. The habitat of the 
Psilotum prothallus is essentially the same as that of Tmesipteris — in wet soil about 
one inch below the surface. As we shall see from the following description, the 
prothalli of these two plants are remarkably alike. 
A description of the spore and its germination has already been given by 
Mr Darnell-Smith, # and a repetition of these stages is not necessary. I will 
only add that I have carefully examined Mr Darnell-Smith’s specimens, and 
fully accept his interpretation. There are, however, other points of detail which 
need amplification, especially in regard to the development of the antheridia and 
archegonia. 
The general form of the prothallus is very like that of Tmesipteris. In nearly 
all the specimens found, however, the prothallus was larger and thicker than those 
* See preceding Paper, The Gametophyte of Psilotum , by G. P. Darnell-Smith. 
