156 
SHI? UBS AMD TREES 
leaf-stalks. Flower-buds also are often laid down in autumn, 
as seen in Prunus or Rhododendron. 
The same principles apply to the resting-bud formation 
in desert plants, &c. The reserves necessary for the re- 
starting of growth are usually stored in the living cells of 
the stem. 
The winter-buds of most trees and shrubs have cha- 
racteristic sizes, shapes, colours, &c. ; tables have been 
constructed for the determination of their genus and species 
from these characters 1 . 
The fall of the leaves is usually effected by the formation 
of an absciss layer at the base of the stalk, cutting off the 
leaf-tissues from those of the stem by a development of 
cork (bark). This splits down the middle and leaves one 
half upon the stem, where it forms the leaf-scar covering 
the wound. Leaves thus cut off are termed articulate; 
those not thus cut off are non-art iculate, as in the oak, 
whose leaves remain hanging upon the twigs most of the 
winter. The formation of the absciss layer is a definite 
vital phenomenon ; broken branches, being deprived of 
their water supply and thus killed, do not form absciss 
layers. 
Whilst in herbaceous plants all or most of the branch 
buds usually develope at once, this is not the case in trees. 
A number of the buds remain dormant and start into 
growth if the others be injured. Adventitious buds are 
often formed on the stems of trees and give rise to the 
leafy twigs seen on the trunks of elms, &c., and to the 
flowers borne on the trunks of many tropical trees, e.g. 
Ficus, Theobroma, Averrhoa. The witches' brooms so often 
seen in birch and other trees — bird’s-nest-like bunches of 
twigs — are due to adventitious branching stimulated by the 
growth of a parasitic fungus (Aecidium) in the stem. Simi- 
larly the crown of branches of a pollard ash or willow arises 
from adventitious buds formed at the cut surfaces. At a 
wounded surface the cambium or formative tissue gives rise 
to a mass of callus , new cellular tissue covering the wound, 
and in this buds may develope. 
1 Pliiss, Unsere Bawne nnd Straucher , Freiburg, 1894; Schneider, 
Dendrologische Winter studien, Jena, 1903; Walser, Der Baum im 
Winter , Bern, 1894; Foerste in Bot. Gazette, 1892. 
