KNOWLEDGE OF THE MUTATING OENOTHERAS. 21 



hybrids it is particularly important that the hybrid offspring should be compared 

 with each other, and with their parents in every stage of their ontogeny, and not merely 

 in the end stages. It is also essential that care be exercised to compare corresponding 

 stages of the rosettes, for misleading conclusions may arise from a comparison of 

 rosettes of different ages. Nowhere is the necessity for taking account of all the onto- 

 genetic stages of organisms more apparent than in such forms as the Oenotheras, 

 where a number of different types of leaf succeed each other in the development of 

 the individual. Indeed, in Oenothera, from the first seedling leaf to the fully 



developed rosette and the mature flowering plant, there is a constant succession of 

 changes in leaf-shape. The cotyledons also pass through a regular series of changes in 



shape and size after they first appear. 



The following series of photographs is offered in proof of some of these statements. 

 They are selected from a much larger number, which have accumulated in connection 

 with the records of my experiments. PL 1. fig. 1 shows a pan of young seedlings of 

 0. Lamcircldana. They are very uniform, but are developing at different rates. 

 T'igs. 25, 21, and 15, which show respectively 0. rubricalijx, 0. hrevistylis, and 0. laevlfolia 

 at the same age, should be compared with this. The comparison will show that even the 

 young seedlings are sharply distinct from each other in shape of leaf-blade and length 

 of petiole. The crinkling of 0. Lamarcklana may appear in even the earliest leaves 



following the cotyledons. 



It should be pointed out that 0. Lamarckiana is by no means the only Oenothera 

 having crinkled leaves. An equal amount of crinkling is to be observed in a race of 

 O. biennis from the Chelsea Physic Garden, which I have grown for several years. 

 Such mutants as O. lata and 0. nanella show as much or more crinkling than 

 O. Lamarckiana^ while nearly all the mutants (even 0. laevifolla) show more or less 



of 



The suggestion of DeVries (1909, i. p. 309) that the crinkling is due to faulty 

 correlation between the growth of the veins and of the areas of mesophyli between them, 

 appears to be correct, and is supported by the fact (referred to elsewhere in this paper) 

 that in certain dwarf plants the reduction in the dimensions of the vessels of the stem 

 is more or less independent of the change in size of the cortical cells, 



PL 1. fig. 2 is a photograph of a rosette of O. Lamarckiana about four months old— 

 L e., four months after the seeds were sown. Pig. 3 shows a similar rosette of 

 O. Lamarckiana at about the same stage of development, grown in heavy clay soil. 



Under these conditions the development of the rosette is slow, and the various types of. 

 rosette-leaves can be carefully followed as they appear. In this way it is found that 

 three distinct types of rosette-leaf successively make their appearance :— (1) Narrow type 

 of leaf, smooth, entire and oblong, with rounded tip : two leavea of this type are 

 present in fig. 3; this is followed by (2) a cycle of three leaves, broad, nearly smooth, 

 with very obtuse tips, narrowed abruptly to the petiole, longer than (1) ; then after one 

 or two transition leaves appears (3) the type of leaf of the mature rosette. These are 



