232 SUMMARY OF CURRENT RESEARCHES RELATING TO 



calcium pectate. The presence of this membrane, together with the 

 fact that the soil particles are held to it by a pectin mucilage, accounts 

 for the high efficiency of the root-hair as an absorbing organ. 



Concentric Xylem in Dicotyledons.* — J. Daniel pubHshes a series 

 of papers dealing with the concentric layers of secondary xylem in 

 Dicotyledons. The following are the chief conclusions : — There is a 

 close connexion between the conditions of life, the rhythmic develop- 

 ment, the organography, and the structure of the secondary wood in 

 Dicotyledons. External morphological differences often correspond to 

 internal difference in the structure of the xylem, and usually result from 

 the same cause. Modes of branching, difference in food-supply, position 

 and time of appearance of the different organs, etc., react upon the 

 xylem structure and symmetry ; it is often possible to foretell such 

 alterations in structure, and even to produce them artificially. Her- 

 baceous plants of rosette habit have characteristic xylem resulting from 

 peculiarity of physiological conditions, but having no connexion with 

 the age of the plant. In trees the relation between the age and the 

 number of concentric layers of wood can be traced for a short time in 

 the short branches, and for a longer time in the main branches and 

 trunk, but after a certain time it becomes impossible to trace any such 

 relationship even with the aid of a Microscope. It is possible to 

 increase or decrease the number of concentric xylem rings by artificial 

 means ; in nature this may occur accidentally. It is not possible to 

 determine the age of a herbaceous plant by examining transverse 

 sections of its root or stem ; it is only possible to do so with trees 

 during a regular and active period of growth, and when it is certain 

 that there has been no violent disturbing factor ; when the old trees 

 have been felled it is impossible to determine their age. The author 

 claims that as his results are based upon experience and fact, not upon 

 hypotheses, they may be of practical use to the gardener, the agricul- 

 turalist, and the cultivator of trees. 



Reproductive. 



Embryo-sac Development.t — P. Baranow has studied the develop- 

 ment of the embryo-sac in Spirmitkes australis and Serapias pseudo- 

 cordigera. In Spiranthes the numerous ovular filaments are composed of 

 single rows of similar sub-epidermal cells. At an early stage the apical cell 

 begins to enlarge, and ultimately becomes the mother-cell of the embryo- 

 sac. Division of the nucleus is succeeded by division of the cell itself 

 into two daughter-cells ; the nucleus of each of these cells contains only 

 half the number of chromosomes found in the mother-cell. The upper 

 daughter-cell soon perishes, while the lower one greatly enlarges and 

 again divides, the nuclear division being homotypic in character ; in 

 this case also the upper daughter-cell perishes, while the lower cell 

 forms the mother-cell of the embryo-sac. Simultaneously with these 



* "Rev. G6n. Bot., xiviii. (1916) pp. 97-115, 133-49, 185-92, 204-20 (8 pis. and , 

 63 figs.). 



t Bull. Soc. Imp6r. Nat. (Moscow), xxix. (1915) pp. 74-92 (29 figs.). 



