370 Causes and Course of Organic Evolution 



growth that soon prevails over apical growth in the former, 

 as compared with rapid and continued apical growth, followed 

 by growth cessation, in the latter. This growth tendency 

 longitudinally, especially in leaves, is to be explained and 

 became accentuated, as Henslow has pointed out, by increased 

 adaptation to a moist environment. We would further sug- 

 gest also shade surroundings as an added factor. 



Now in such rapid elongation from a growth apex, along 

 with frequent bending downward of the radicle when the seed 

 fell on its side, a slight but appreciable difference in length of 

 one cotyledon over another would almost inevitably occur. 

 The longer cotyledon, by its absorptive apex, would reach 

 and digest more abundant reserve food. Through long con- 

 tinued action thus, a slow but sure absorption of one cotyledon, 

 or possibly displacement of it through longitudinal growth 

 between it and the cotyledon retained, would be effected, while 

 the remaining one, or the lower one of the two, would grad- 

 ually encroach on the stem apex, as being increasingly in direct 

 line with the lengthening radicle. The diffuse intercalary 

 growth and frequent absorption-activity over both of the 

 external faces of the cotyledons, in cordaital, in gnetal, and in 

 primitive dicotyledonous embryos, would ensure a more balanced 

 cotyledonary growth, and thus a retention of both seed-leaves. 



The comparative stem structure of dicotyledons and mono- 

 cotyledons is characteristic. But, as Henslow and Sargent 

 have pointed out, various exceptions in the growth of both 

 types are met wdth. Thus amongst the former the nym- 

 phseaceous, the piperaceous, and the curvembryonal aUiances 

 show marked resemblances in their stems to those of mono- 

 cotyledons. Conversely not a few of the simpler monocoty- 

 ledons show one or more tolerably well-marked bundle rings. 



In the latter also marked apical and longitudinal growth 

 of the foliage leaves, in relation to shade and to moist envir- 

 onment, have had much to do, in the writer's estimation, 

 with the breaking up of the bundle-ring, ^^'ith apical increase 

 in the stem, with irregular scattering of the bundles, and Tvith 

 increase in number of these bundles. For the extensive -wind 

 or water strains caused through the leaves on the stem would 



