688 



SCIENCE. 



[N. S. Vol. XIV. No. 357. 



water on a land area has been solved more 

 satisfactorily than in the case of the groups 

 that preceded them." 



By the formation of the flower and seed 

 the Angiosperms freed themselves from the 

 risks which attend sexual reproduction in 

 heterosporous Pteridophytes by providing 

 a special nidus for the development of the 

 germ and thereby rendering it directly in- 

 dependent of the presence of water. The 

 tegumentary system of the Angiosperm 

 ovule has for its primary function the con- 

 veyance and storage of water for the em- 

 bryo and in addition serves as a food reser- 

 voir. The function of the ovular tegmina 

 cannot now be regarded as of so much impor- 

 tance in the reproductive act as was for- 

 merly the case, and the existence of haus- 

 toria which penetrate them, either from the 

 embryo itself or the embryo-sac, point 

 clearly to their function as reservoirs of 

 food and water. In passing it was pointed 

 out that the classification of the Dicotyle- 

 dons into Unitegminese and Bitegminese 

 proposed recently by van Tieghem seems to 

 rest upon an insecure foundation, since all 

 the genera in certain families, such as the 

 Eosacese and Eanunculacese are not alike 

 in respect to the number of teguments. 



And it is not only to this development of 

 special water reservoirs for the ovule that 

 the Angiosperms owe their advantage as a 

 land-type, but in two features of their water- 

 carrying system they are greatly superior 

 to the older types. No one will deny that 

 their general monostely is a more perfect 

 arrangement for water carriage in a massive 

 plant than is polystely, nor is there doubt 

 that the vasa which are conspicuous char- 

 acteristics of the Angiosperms are more 

 favorable for a rapid transport of water 

 than are tracheids. 



Passing on to a consideration of the differ- 

 entiation of the Angiosperms into classes, 

 Professor Balfour discussed the new class 

 proposed by van Tieghem, that of the 



Liorhizal dicotyledons, and came to the 

 conclusion that the two recognized fam- 

 ilies included in the class, the Nymphse- 

 acese and Graminese, do not present suf- 

 ficiently distinctive characters to warrant 

 their separation from the already estab- 

 lished classes. The most recent observa- 

 tions on the embryogeny of the Nym- 

 phseacese seem to indicate that the apparent 

 dicotyledonous nature of the embryo is due 

 to the splitting of a simple cotyledon, and if 

 this be correct the order is most properly 

 assignable to the monocotyledons and the 

 structure of the root-tip upon which von 

 Tieghem lays so much stress is what might 

 be expected. The idea that the epiblast of 

 the embryos of the Graminese represents a 

 second cotyledon. Professor Balfour is in- 

 clined to dispute, and points out that in 

 any event its occurrence is not universal in 

 the order, since it is present in Triticum 

 and absent in Secale, present in Elymus and 

 absent in Hordeum. The evidence as to 

 the morphological significance of the struc- 

 ture is at present too obscure to warrant its 

 being taken as a basis for the separation of 

 the Graminese from monocotyledons. 



Becognizing then but the two classes, 

 Monocotyledons and Dicotyledons, the 

 lecturer stated that if he were to express an 

 opinion as to their phyletic relationship it 

 would be that they had arisen on separate 

 lines of descent. The Dicotyledons are 

 by far the more adaptive and progressive, 

 though this does not necessarily imply their 

 more recent origin, and the advantages 

 which they present over the Monocotyle- 

 dons in their free internodal growth and 

 copious root system as compared with the 

 contracted stem growth and arrested root 

 system of the latter, are but a carrying out 

 of the structure of the embryo with its 

 terminal plumular and root buds and its lat- 

 eral cotyledons, so markedly different from 

 what obtains in the Monocotyledons, in 

 which the cotyledon is terminal, the plumu- 



