September 28, 1900.] 



SCIENCE. 



469 



an aggregate of cells, but as a more or less 

 septated mass of protoplasm : the synthetic 

 standpoint of Schwann has been replaced 

 by one as distinctively analytic. 



Time does not permit me to do more 

 than mention the important discoveries 

 made of late years, mainly on the initiative 

 of Strasburger, with regard to the details 

 of cytology, and especially to the structure 

 of the nucleus and the intricate dance of 

 the chromosomes in karyokinesis. Indeed, 

 I can do but scant j ustice to those anatomical 

 discoveries which are of more exclusively bo- 

 tanical interest. One important generaliza- 

 tion which may be drawn is that the histo- 

 logical differentiation of the plant proceeds, 

 not in the protoplasm, as in the animal, but 

 in the cell-wall. It is remarkable, on the 

 one hand, how similar the protoplasm is, 

 not only in different parts of the same body, 

 but in plants of widely different affinities ; 

 and, on the other, what diversity the cell- 

 wall offers in thickness, chemical composi- 

 tion, and physical properties. In studying 

 the differentiation of the cell-wall the bot- 

 anist has received valuable aid from the 

 chemist. Research in this direction may, 

 in fact, be said to have begun with Payen's 

 fundamental discovery (1844) that the 

 characteristic and primary chemical con- 

 stituent of the cell- wall is the carbohydrate 

 which he termed cellulose. 



The amount of detailed knowledge as to 

 the anatomy of plants which has been ac- 

 cumulated during the century by count- 

 less workers, among whom Mohl, ISTaegeli, 

 Unger, and Sanio deserve special mention 

 as pioneers, is very great — so great, indeed, 

 that it seemed as if it must remain a mere 

 mass of facts in the absence of any recog- 

 nizable general principles which might 

 serve to marshal the facts into a science. 

 The first step towards a morphology of the 

 tissues was Hanstein's investigation of the 

 growing point of the Phanerogams (1868), 

 and his recognition therein of the three 



embryonic tissue-systems. This has lately 

 been further developed by the promulgation 

 of van Tieghem's theory of the stele, which 

 is merely the logical outcome of Hanstein's 

 distinction of the plerome. It has thus be- 

 come possible to determine the homologies 

 of the tissue-systems in different plants and 

 to organize the facts of structure into a 

 scientific comparative anatomy. It has 

 become apparent that, in many cases, dif- 

 ferences of structure are immediately trace- 

 able to the influence of the environment ; 

 in fact, the study of physiological or adapt- 

 ive anatomy is now a large and important 

 branch of the subject. 



The study of Anatomy has contributed 

 in some degree to the progress of systematic 

 Botany. It is true that some of the more 

 ambitious attempts to base classification on 

 Anatomy have not been successful ; such, 

 for instance, as de CandoUe's subdivision 

 of Phanerogams into Exogens and Endo- 

 gens, or the subdivision of Cormophyta 

 into Acrobrya, Amphibrya, and Acram- 

 phibrya, proposed by Unger and Endlicher. 

 Still it cannot be denied that anatomical 

 characters have been found useful, if not 

 absolutely conclusive, in suggesting af- 

 finities, especially in the determination of 

 fossil remains. A large proportion of our 

 knowledge of extinct plants, to which I 

 have already alluded, is based solely upon 

 the anatomical structure of the vegetative 

 organs ; and although afQnities inferred 

 from such evidence cannot be regarded as 

 final, they sufiSce for a provisional classifi- 

 cation until they are confirmed or dis- 

 proved by the discovery and investigation 

 of the reproductive organs. 



PHYSIOLOGY. 



The last branch of the botanical science 

 which I propose to pass in review is that of 

 physiology. We may well begin with the 

 nutritive processes. At the close of the 

 eighteenth century there was practically no 



