64 SUMMA.RY OF CURRENT RESEARCHES RELATING TO 



BOTANY. 



GENERAL, 



Including the Anatomy and Physiology of Seed Plants. 



Structure and Development. 

 Veeretative. 



Wood of Deciduous Trees.* — P. Jaccard Las studied the chano'es 

 brought about in the wood of dorsi-ventral branches of deciduous trees, 

 by subjecting the branches to alternative pressures, varying in duration, 

 direction, and intensity. The anatomic characters of the wood were 

 compared with those of the wood of normal branches, and the following 

 results were obtained : — Under the influence of longitudinal tensions and 

 compressions the wood of the upper and lower surfaces differed greatly 

 in morphology and chemical composition. An analogous difference may 

 also be brought about by other causes, such as geotropism, or by any 

 force which creates a longitudinal tension or pressure. The difference 

 is characterized by the formation of typical tension-fibres and compres- 

 sion-fibres. Such fibres are only formed in trees, and are never found 

 in shrubs or woody annuals ; they must not be regarded as specific in 

 character, since they only occur in response to external mechanical 

 pressure. It is not possible to ascribe to them the value of an acquired 

 character which can be transmitted by heredity ; neither have they any 

 phylogenetic significance. 



Reproductive. 



Embryogeny of Phaseolus.f — M. M. Brown has studied the 

 development of the embryo-sac and of the embryo in five varieties of 

 Fhaseolus vulgaris. The chief features of interest are as follows : — At 

 an early stage in the growth of the ovule, a large hypodermal cell takes 

 on the functions of a macrospore-mother-cell, and when fully grown 

 forms part of the third or fourth layer from the micropylar end of the 

 nucellus ; ultimately an axial row of three macrospores is formed, the 

 innermost of which forms the embryo-sac. All the nucellar tissue is 

 absorbed except that at the base of the sac, where it undergoes some 

 changes, but finally disappears. The polar nuclei approach each other 

 at an early stage, and after remaining in contact for some time, finally 

 fuse just below the egg. At the time of fertilization the three antipodal 

 cells disappear, but the synergids form a conspicuous filiform apparatus. 

 The pro-embryo is composed of three cells, of which the two basal ones 



* Rev. G6n. Bot., xxix. (1917) pp. 225-43 (2 pis.). 



t Bull. Torrey Bot. Club, xliv. (1917) pp. 535-44 (2 pis.). 



