NOTES ON ROOT-HAIRS AND ROOT GROWTH. 
185 
On the whole, the results of the four sets of observations (two 
for the Mustard, two for the Cress) are quite as uniform, if not 
more so, than is ordinarily the case in similar experiments. 
In the case of No. 1, the relatively impenetrable character of 
the clay is well shown in the table ; the strongest seedlings did 
indeed manage to thrust their radicles through it, and to reach 
the sides of the pot, and then, and not till then, did they pro¬ 
duce any root-hairs; so that the mere presence of a solid 
obstacle is not of itself sufficient, as stated by Chatin, to 
account for the presence of the hairs. 
The general result was so irregular that, on the second 
examination, it was found impracticable to get any fair average; 
some of the seedlings were dead, many were weakly, a very few 
had managed to penetrate the clay to any distance. The 
relative barrenness of stiff clays in this way receives some 
illustration. 
In the case of Nos. 2 and 8 (pebbles and ballast), the con¬ 
ditions were nearly the same; water and air could be admitted 
freely, and the roots could easily penetrate the interstices of the 
stones, and avail themselves of the thin films of water adhering 
to the stones. The result is shown in the high average vigour 
of the seedlings, and in the length of the radicles. It was 
noticeable also that root-hairs were plentifully produced under 
the lea of the stones. 
In 4 (garden soil) a high average degree of vigour was 
observed in both the Mustard and the Cress; but in the former 
the relative proportions of tigellum and radicle were more 
uniform than in the case of the Cress. The roots of the Cress 
were more highly developed as to size in this pot than in any 
other of the Cress series. In the case of the Mustard, it will be 
observed that the roots in No. 4 appear in the diagram to have 
been longer in February than they were in the corresponding 
pot examined a fortnight later; but this appearance is false, and 
arises from the fact that the dot is accidently placed too low in 
the column and the asterisk too high. 
In number 5 (flannel) we have a similar discrepancy, the 
total length of the Mustard seedlings and of their radicles being 
noted as greater in the first batch than in the second. No such 
discrepancy is observable in the case of the Cress grown in the 
same pot. The seedlings of the Mustard were very much 
