S04 SECTIONAL TRANSACTIONS— K. 



justification for this neglect existed at a time when Httle knowledge of floral 

 anatomy was available and when reliance had to be placed upon the general 

 rule of the alternation of successive whorls as the sole guide to the solution 

 of the problems of floral morphology. 



The inadequacy of the basis of many of the solutions adopted is revealed 

 increasingly by a study of floral anatomy. It has become evident that the 

 traditional interpretation of that complex organ, the syncarpous gynaeceum, 

 does not accord with the evidence, and that, even respecting the perianth and 

 andrcecium, accepted views must be revised, as in Cistaceas, Amarantacese, 

 Hypericum, Soldmiella, Saraca, Cucurbita and many other types. 



Descriptions of the flower in the more detailed accounts have mostly been 

 written before the advent of this new knowledge. Hence they depict, but 

 they rarely analyse. They describe the visible, but seldom indicate how the 

 variations observed have been effected. In order that accounts which may 

 appear in the future should present a more complete picture of the flower 

 and of its history, it is now greatly to be desired that the workers in the two 

 fields of systematic botany and anatomy should co-operate in framing and 

 adopting a terminology which embraces the known facts. 



Prof. F. E. Weiss, F.R.S. — Apparent reversions in variegated hollies (10.40). 



White and green margined holly leaves are the foHage of periclinal 

 chimasras in which the core or centre of the leaf is surrounded usually by 

 two layers of cells of diff'erent colouring. Thus in the white margined 

 leaves the two peripheral layers of cells are devoid of chlorophyll, while 

 central portions of the mesophyll contain abundant green colour. From the 

 general laws of leaf development we may assume that three layers of cells of 

 the stem apex are concerned in the development of the holly leaf, and that 

 in the white margined leaves the bulk of the leaf tissue arises from the 

 innermost of these three layers which is potentially green, while in the 

 thinner margins of the leaf the tissues have arisen entirely from the two 

 outermost colourless layers of the stem apex. 



On holly-trees with white margined leaves certain branches may be pro- 

 duced at the base of the plant which bear completely green leaves. Such 

 branches or suckers arise from the normally green stock upon which the 

 variegated holly has been grafted. More commonly twigs bearing com- 

 pletely white leaves are observed on the branches. These have been con- 

 sidered to be reversions to the colourless component of the chimaera. 



A microscopic examination of such white branches has, however, shown 

 that they are not colourless throughout, but that they have a central green 

 core, as can be seen from the inner cortical cells which contain abundant 

 chlorophyll. The colourless mantle by which the green core is surrounded 

 may be six to ten layers in thickness in the mature twig, and we may assume 

 that these have arisen by division of the second layer of colourless cells. 

 We are dealing therefore with a pericHnal chimasra in which the mantle 

 consists of more than two layers of cells. It may be trichlamydeous or, as 

 indicated, have an even greater number of peripheral cells. There is indeed 

 often a considerable irregularity in the number of layers of colourless cells, and 

 in some cases white margined and completely white leaves are found on the 

 same twig, the former arising from dichlamydeous, the latter from trichla- 

 mydeous portions of the stem apex. The arrangement of the number of 

 colourless cells in the stem may tend to become somewhat sectorial. 



What has been said of the white margined hollies is true mutatis 

 mutandis of the green margined forms. It is also true of other plants with 

 periclinally variegated leaves such as Euonymus, Eleagnus, etc. 



