427 



fa<jt, the cork outgrowths can arise in the same leaf in different layers (they 

 usually occur in the sub-epidermal layer) and can have a different develop- 

 mental course (Bachmann). 



The external appearance of these cork formations on leaves, occurring 

 on gymnosperms, monocotyledons, and dicotyledons, is very different. 

 Sometimes there are small cones, sometimes sheets of cork, or strips of 

 considerable extent. At times the cork excrescences, however, lead to the 

 formation of holes which penetrate through the whole leaf (Ilex, Zamia, 

 Ruscus, Camellia axillaris, Peperomia obtusifolia. Eucalyptus Gunni and 

 E. Globulus, etc.). This perforation begins as yellowish points. In leaves 

 with large intercellular spaces the cork formation is preceded by a growth 

 of the parenchyma cells, in such a way that the intercellular spaces are filled 

 by outpushings of the cell walls. If the cells with somewhat thicker walls 

 in the rows of cork cells are changed by repeated division, the cell walls 

 lose their original thickness. Frequently also the cork cells undergo a 

 subsequent elongation after they have split the epidermis ; the outer ones 

 are stretched first. 



In Zamia integrifolia, brown stripes, running parallel with the veins, 

 are found on leaflets, splitting later into pieces or tearing down the whole 

 length. These stripes are cork tissue which are not produced sometime 

 after the leaflets have been torn and, thereby, representing wound cork, but 

 are structures formed even embryonically in the younger leaf. Cork ex- 

 crescences appear on both sides of the older leaves of Dammara robusta, 

 but especially on the upper side, remaining usually small and flat. When 

 young, they form small round spots on the green leaf surface and later be- 

 come brown, when they are raised like little mounds. Finally, the epidermis 

 and the immediately adjacent cork layers rupture. In Araucaria Cunning- 

 hami and more rarely in A. Bidwilli, small cork mounds may be found on 

 older leaves of the previous year which can coalesce into ridges. In 

 Sciadopytis verticillata and Cryptomeria japonica small cork warts occur 

 at times also on older leaves ; such structures may be recognized more fre- 

 quently (but usually only on the underside) on the broad leaves of Sequoja 

 sempervirens. In commercial horticulture, small point-like cork warts in 

 Cyclamen persicum form a blemish as do also the chart-like etchings on 

 the upper side of leaves of Pelargonium peltatiim and in different kinds of 

 foliage Begonias. These cork outgrov/ths appear, so far as observed, only 

 in moist greenhouses and hot beds. 



Among the monocotyledons, Clivia Gardeni, Hook and Clivia nobilis, 

 Lindl., Pandanus reflexus, Dichorisandra oxypetala, Billbergia iridifolia. 

 Vanilla planifolia, and other orchids exhibit cork structures which penetrate 

 into the leaf. The cork excrescences on the leaves do not occur in the same 

 amount in all specimens, nor to equal extent on all the leaves of the same 

 plant, nor are the appearances constant each year. It must be concluded 

 from this that special conditions cause this development of cork structures. 

 So far as experience shows, they are due to an excessive atmospheric 



