290 PLANT GROWTH SUBSTANCES 



In land plants a similar phenomenon may be observed. Palmate leaves 

 often have different degrees of division in the same plant. In general the 

 older leaves are more divided than the younger ones up to a certain age 

 when the division decreases again. In this case there also seems to be a 

 differential between vein growth and mesophyll growth; the larger the 

 quotient vein/mesophyll the deeper incised the leaf will be. Good 

 examples of such a series in one plant are given by Krenke (16, figures 

 1 12-120). Another extreme case is Begonia carolinaefolia (7). The figures 

 of Krenke (15, figures 17-19) for Broussonetia papyrifera, are also good 

 examples of differential growth. Goebel (8, page 37) already gives a 

 scheme explaining various leaf forms by differential development of the 

 various regions of the leaf primordia. \^elenovsky (32) also mentions in 

 general: "Es geschieht auch nicht selten dass die Spreite an der Rippe 

 teilweise oder ganz an einer oder an beiden Seiten verschwindet." 



In the case of bud scales (8,32), we find nice transition series between 

 typical scales completely lacking in leaf blade and normal leaves. Since 

 the bud scales are considered as transformed stipules rather than leaves, 

 we can suppose that at the time of their development there were different 

 amounts of stipule and leaf growth factors present. This conclusion 

 is strengthened by the observation that under certain conditions organ 

 primordia which normally would have developed into scales can give 

 rise to more or less normal leaves (8, page 428). Such observations are 

 frequently made on the second bud scale of pea seedlings. A quantitative 

 expression of such differences (heteroblastic development) was recently 

 attempted by Ashby (2), who did not present a physiological explanation, 

 but only a graphical or mathematical presentation. 



Differential vein-mesophyll development is common in leaves on 

 flowering branches. Often leaf shape changes when the flowering condi- 

 tion is attained. Typical and often quoted examples are the difference 

 in leaf form on flowering and vegetative branches of Hedera helix and 

 of Campanula rotundifolia. In most other plants the leaf implanted just 

 below a flower stand has reduced mesophyll development. 



Light. — The effect of light on growth is complex. In the first place 



dicated by the following quotation from Velenovsky: "Die Spaltung des unter- 

 getauchten Blattes in lineale Abschnitte ist bei den dikotylcn Aiten eine ver- 

 breitete Erscheinung." J. Velenovsky, Vergleichende Moiphologie der Pflanzen 

 (Prague, 1905), 413. 



