138 



THE AMERICAN BOTANIST 



were not real hybrids. The real explanation was still to be 

 found. This was accomplished by Professor Erwin Bower of 

 Berlin who discovered the key to the situation in his studies of 

 the tissues of a variegated geranium. Plants which bear green 

 leaves with white borders are well known. Such leaves are due 

 to the fact that the green central part of the leaf tissue is 

 covered by a layer of colorless tissue twO' or more cells in 

 thickness, the thin edges of the leaves as a consequence being- 

 made up of colorless cells exclusively. Thus the colorless tissue 

 covers the leaf as a glove covers the hand. Now it is supposed 

 that in the growing point from which such leaves spring, a 

 layer of colorless cells covers the green tissue beneath and a leaf 

 originating from such growing point will consist of green cells 

 within and colorless cells without. Likewise in the formation 

 of a terminal bud at the juncture of the two parent tissues of 

 the graft, the tissues of one parent, whether stock or cion, by 

 growing a little faster, may soon cover that of the other parent 

 and the result will be a shoot composed internally of the cells 

 of one parent with a covering made up of cells of the other 

 parent. This results in a periclinal chimera. If, on the other 

 hand, one half a sector of the terminal bud be formed wholly of 

 cells of one parent, the parts of the resultant shoot resembling 

 the respective parents can be separated logitudinally and the 

 result is a sectorial chimera. 



Winkler's apparent hybrids, were therefore, periclinal 

 hybrids. For example, his Solaniiin tnbingensc was a periclinal 

 chimera in which the inside of the plant consisted of nightshade 

 overlaid with one layer of tomato tissue, since the seeds pro- 

 duced gave pure nightshades. It will be remembered that the 

 germ cells of spore-bearing tissues are derived from the sub- 

 epidermal cells. On the other hand, Solaninn pro feus resemb- 

 ling more closely the tomato, and whose seeds produced pure 

 tomato plants, is a periclinal chimera in which the interior is 

 tomato with a covering of nightshade two cells in thickness. 



