376 MR. T. W. WOODHEAD ON THE 
experiments, that the development of mechanical tissues is not 
so indueed; and he failed to produce either increase in the 
ability to resist mechanical pulling, or any thickening of the 
tissues, as the result of a gradual increase in the pull exerted 
upon young stems. The results he produced were inconstant— 
sometimes a thiekening was produced, sometimes not. We 
must look, therefore, to some other (probably a series of inter- 
acting) eauses for a full explanation of the development of 
stereom, 
Pteris aquilina. Rhizome. 
Many observations have been made and much has been written 
on the presence or absence of Pteris on caleareous soils, and the 
influences of the physical and chemieal nature of the soil on its 
distribution and structure. In West Yorkshire it shows a 
distinct preference for sandstone soils, but, as indicated by 
Lees (55) and others, it is by no means absent from limestone; 
while More (70) includes it in the group “ Calcifuge B," that is, 
not infrequent on lime soils, but invariably in greater abundance 
and luxurianee on soils from which lime is absent. More 
recently its distribution from this point of view has been dealt 
with by Gillot & Durafour (34). In the Huddersfield district 
ealeareous soils are absent, therefore lime cannot be one of the 
factors influencing its distribution within this area. 
Masclef (64) examined the rhizomes of Bracken growing 
respectively in clayey and calcareous soils on the escarpments 
at Rogerville and Harfleur and also on the granite rocks at 
Cezembre. He found that in siliceous soils they present the 
normal structure; in soils impregnated with lime the scleren- 
chyma was greatly developed, the outer band of reserve con- 
junctive tissue being reduced to half its normal diameter; 
whilst in pure chalk the sclerenchyma was increased to such 
an extent as to reduce the reserve tissues to a very narrow 
band. These changes he attributes to the chemical influence 
rather than to the physical nature of the soil. He found that 
specimens collected on soils containing 5 to 7 per cent. of lime 
showed structures between those growing on pure chalk and 
those growing on exclusively siliceous soils. No such quantity 
of lime could be expected in the soils in the Huddersfield 
district, and careful analyses of many samples showed them to 
contain from ‘02 to *0£ per cent. of lime; yet an examination 
