Problem of Xeromorpky in Marsh Plants . 849 
intermediate forms is determined by heredity, or how far it is attributable 
to individual variation, is at present problematical. 
Goebel, in his interesting researches on the various forms of leaves in 
Campanula rotundifolial found that leaf form depended largely on illumina- 
tion. In nature the seedling leaves, as well as the first leaves formed on 
the lateral runners, are always of a petiolate, rotund type. Linear leaves, 
on the other hand, are only formed on the erect flowering shoots. By 
lowering the light-intensity, rotund leaves could be induced to form on the 
flowering shoots in the place of linear ones. But the converse did not hold. 
Even the strongest illumination, whether continuous or intermittent, was 
powerless to hinder the formation of rotund leaves on either the seedlings 
or the runners. Goebel regarded the rotund form of the leaf in these cases 
as hereditarily fixed. 2 
Now the glabrous leaves of Spiraea Ulmaria are formed in nature 
under precisely the same conditions as the rotund leaves of C. rotundifolia , 
and the hairy ones correspond in position, &c., to the linear leaves of 
Campanula . 3 Further, all attempts to induce the formation of hairs either 
on seedling Spiraeas or on the early spring leaves of mature plants have 
been unsuccessful. It is therefore probable that glabrousness is a character 
hereditarily fixed in these leaves, just as Goebel supposes roundness to be 
in the corresponding leaves of Campanula. 
{b) Individual variation. One of the most obvious expressions of 
this is seen in the variable number of glabrous leaves formed on a shoot. 
Usually the number varies from three to five, though in exceptional cases 
as many as twelve glabrous radical leaves have been observed. Again, in 
most cases one or more hairy radical leaves are produced before the erect 
flowering shoot begins to elongate. But individuals are sometimes found 
(see p. 839) in which even the lowest one or two cauline leaves are glabrous 
or nearly so. 4 As these plants occur side by side . with normal ones, 
it seems probable that it is simply a case of individual variation. 
( c ) Effect of external conditions. This has already been discussed at 
length, and the conclusion arrived at that hair production is promoted by 
marked periodic fluctuations in the turgor of the hair-producing cells. It 
has also been shown that Spiraea is plastic only to a limited extent in regard 
to hairiness. Marked changes in the external conditions are required before 
any visible response can be obtained. This plasticity is less than that of 
Campanula in respect of leaf form, or than the plasticity with regard to hairi- 
ness in such plants as Mentha aquatica , &c. The stimulus, then, which 
1 Goebel (’96), (’08), and other papers. 2 Goebel (’96), pp. 8 - 9 . 
3 There is frequently a slight approach even in the matter of form ; see Text-fig. 5 and 
PI. LXXXI, which show the tendency in Sph'aea to form rounder leaflets below and narrower ones 
above. This is a common phenomenon in plants. 
4 And this in localities in which var. denudata does not occur, so that it can scarcely be a case 
of hybridization. 
3 K 
