842 Yapp . — Spiraea Ulmaria , A., and its Bearing on the 
continue until only the tissue immediately surrounding the larger veins 
remains green ; 1 finally this too may wither and die. 
Further, a similar localization is seen in other connexions. Thus 
anthocyan in young leaves in spring-time is frequently confined to the 
tissues at the apex of the leaf, round the margins, and between the larger 
veins. 
Lidforss 2 figures leaves of Viburnum Tinus , in which certain portions 
have been killed by frost. These portions are the margins, and to some 
extent the interveinal regions. On one occasion the present author noticed 
an autumn leaf of 5 . Ulmaria which had been injured by frost. Not only 
did the frozen portions show a similar localization in the individual leaflets, 
but the different leaflets exhibited decreasing injury from above downwards 
(cf. the hairiness described on p. 820). 
Again, Professor T. H. Macbride, of Iowa, told me in 1909 that 
Myxomycetes often confine their attacks to just these same regions of a leaf. 
It is obvious that those regions of the leaf lamina which show a special 
tendency to form hairs (in Spiraea ), to wither on exposure to strong winds, 
&c., &c., are physiologically different from the remaining parts. So far as 
their water supply is concerned, it may be pointed out that they are usually 
thinner and more exposed than the parts immediately surrounding the 
larger veins. They are thus more liable to lose water. Further, they are 
themselves just the regions of the lamina which are most remote from the 
water supply coming up from below and passing along the main veins. 
§ it. The Causes of the Formation of Hairs and Palisade 
Cells. 3 
Hairs on aerial shoots and palisade cells have a good deal in common. 
Both are elongated in a direction more or less at right angles to the surface 
of the organ which bears them. The production of both is favoured (at 
least in many cases) by the same external conditions (see § 9). Root-hairs 
cuspidat-um . If in exposed situations, the leaves of this plant are often marked in the spring-time 
with red bands, parallel to the midrib, and therefore crossing the larger lateral veins. Later, these 
red bands may disappear, or, if windy weather ensues, the tissue may wither and actual lesions occur 
(Text-fig. 11, h). This and other Polygonaceae (e.g. Rmnex spp.) are the only instances I have 
seen in which the withered areas cross the larger veins, instead of lying between them. It is to be 
noted that the injury occurs in those parts which are exposed during the unfolding of the revolute 
leaves (Text-fig. 11, j). If withering is delayed until the leaf is mature, it is then localized round 
the margins in the normal manner. 
1 In certain cases, e. g. leaves of Acer, Populus , &c., fallen leaves may be found with the larger 
veins dead and brown, and the interveinal tissues green. This is the exact converse of exposure 
withering. It is due, so Mr. G. Massee informs me, to the action of certain parasitic Fungi, which 
attack only the leaf veins. 
2 Lidforss (’ 07 ), Taf. I. 
3 A short paper on this subject was read at the Portsmouth meeting of the British Association, 
Yapp ( 12 ), p. 565. 
