136 SUMMARY OF CURRENT RESEARCHES RELATING TO 



BOTANY. 



GENERAL, 

 Including the Anatomy and Physiology of Seed Plants. 



^ Structure and Development. 



Veg-etative. 



Abscission in Mirabilis.* — F.E.Lloyd has studied the phenomenon 

 of abscission in Mirahilis jalapa, in order to prove that Hannig is 

 incorrect in his view that in this species abscission is the result of the 

 complete solution and destruction of a layer of tissue. The author finds 

 that " there is no antecedent structural indication of the position of the 

 abscission zone." In young parts one tier of cells gives evidence of 

 physiological activity, while in older parts ten to twelve tiers are involved. 

 The greatest activity is in the uppermost five layers, wiiich constitute 

 the separation-layer and correspond to Hannig's " Losungsschicht." 

 Abscission begins in the innermost part of the cortex, near the base of 

 the internode, and extends inwards towards the pith and outwards to the 

 epidermis ; the uppermost tier of the cells involved changes most rapidly. 

 The cell-walls undergo chemical change, and in the separation-layer the 

 primary and secondary walls are completely digested ; the tertiary walls 

 separate from each other and from the partially altered primary and 

 secondary walls of the next tier of cells. Only the uppermost tier of the 

 separation-layer undergoes complete separation. Starch appears to be 

 the source of etiergy in the process of separation. When separation is 

 complete the protoplasts are left invested by a tertiary membrane which 

 grows independently of other membranes. Neither cytoplasm nor nuclei 

 take part in the degeneration changes, but they exhibit greater physio- 

 logical activity, and are quite normal when separation is complete. 

 There is no loss of turgor. The author concludes that Hannig is 

 mistaken in regarding this as a new type of abscission, since the essential 

 details are the same as those which occur in Gossypium, Aristolochia, etc. 



Australian "Grey Mangrove." f — R. T. Baker contributes a paper 

 dealing with Avicennia q^cwi«Z<.s, popularly known as the "grey man- 

 grove." After a brief discussion of the systematic position of this 

 species, the author proceeds to a description of the leaves, breathing- 

 roots, seed, timber, and bark. The leaves were found to exhibit two 

 remarkable features : 1. Numerous depressions on the upper surface of 

 the leaf, resembling stoniata in appearance, but of diff'erent structure and 

 function ; these are probably contrivances for increasing the area of the 



* Bot. Gaz., Ixi. (1916) pp. 213-30 (1 pi and 2 figs.). 



t Journ. and Proc. Eoy.Soc. New South Wales, xlix. (1916) pp. 257-81 (19 pis.). 



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