Tissue Mixtures 269 



flowers, fruits, and seed. Whether these were really mixtures of tissue, 

 or, in some cases at least, were actual vegetative hybrids, was not clear 

 at first. Baur's (1909) analysis of a Pelargonium with white-margined 

 leaves showed that in this plant both the epidermis and the layer 

 beneath it lacked chlorophyll. This conception of a continuous layer 

 of one type of cells covering a core of another type was applied to 

 Winkler's chimeras, and the latter proved to be periclinal ones. Such 

 forms apparently arise at a place where there is a thin layer of one 

 tissue over the other. A growing point, originating in the deeper tissue, 

 pushes up and carries on its surface one or two cell layers of the other 

 type. From this layered meristem a new shoot is formed. Mericlinal 

 chimeras may thus be converted into periclinal ones. Jorgensen and 

 Crane (1927) repeated Winkler's experiments, using five species of 

 Solarium, and observed in more detail the origin of chimeras. 



In the tomato-nightshade chimeras four different forms were recog- 

 nized, propagated, and even given Latin names. In one there was a 

 single layer of tomato over a core of nightshade; in another, two layers; 

 in a third, one layer of nightshade over tomato; and in a fourth, two 

 layers of nightshade. The reason that there are rarely more than two 

 layers of the outer component is presumably because a new growing 

 point always arises near the surface. 



In Winkler's material it was relatively easy to distinguish the two 

 components of the chimera cytologically, since tomato has 24 chromo- 

 somes (2n) and nightshade 72. When chromosome counts could not be 

 made, the size of the cells (much larger in nightshade) was almost as 

 good a criterion. It was found that the layers could be distinguished 

 at the apical meristem and that they maintained their specific character 

 throughout the life of the plant. When the outermost layer at the grow- 

 ing point was from one partner (tomato, for example) only the epi- 

 dermis of the plant was of that type. When the second layer of the 

 meristem, as well, was from tomato, the two outer layers of the plant 

 were of this type. Occasionally in certain tissues of the mature plant these 

 layers would become somewhat thicker by periclinal divisions and thus 

 include more cells, but this was relatively uncommon. 



In periclinal chimeras (as in all seed plants) the genetic character of 

 the plant is determined by the cell layer just beneath the epidermis. 

 From this layer the sporogenous tissue is formed. The offspring of a 

 chimera, by seed, is therefore identical with the graft partner that con- 

 tributes the subepidermal layer of cells. 



Winkler maintained that two other types of chimeras that he obtained 

 from grafts were true burdos, or vegetative hybrids, in which one layer 

 had arisen by an actual nuclear fusion between cells of the two com- 

 ponent species. The aberrant chromosome counts (cells with neither 24 



