ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 527 



a combination of these two types in the hip of the rose, the fruit of 

 Lonicera xylosteum, the perianth of Iris pseudacorus, and the petals of 

 Cucurbita pepo. The third type occurs in the perianth of TuUpa 

 Gesneriana, the root of the carrot, and the perianth of Maxillaria 

 triangularis. Chromoplastids are always developed from round leuco- 

 plastids or chloroplastids. The angular form is the result of changes 

 in shape of the entire plastid, not of any disruption. No connection 

 can be traced between the form of chromoplastids and the systematic 

 position of the plant in which they occur. 



The albumen of a number of plastids belonging to these three 

 types passes over, in the living cell, partially or entirely, temporarily 

 or permanently, from the living into the crystalline condition. In 

 leucrplastids the albumen is comparatively rarely in the crystalline 

 condition ; the forms observed may be classified under three types : — 

 the fusiform, rod-shape(J, and spherical. The two first types are very 

 unstable. Their chemical composition is always very nearly that of 

 living protoplasm ; they may pass over directly into it without, at 

 least at first, losing their crystalline form. The living portion of the 

 plastid is renewed by the direct transformation of crystallized into 

 living protoplasm. The albumen of chloroplastids is also compara- 

 tively seldom crystalline. That of chromoplastids crystallizes more 

 often. Except in Cucurhita, this takes place, in all the cases ex- 

 amined, before the opening of the flower or ripening of the fruit, often 

 in the very young organ. In the act of crystallization the colouring 

 substance is either mechanically inclosed or is less often thrown 

 out. These crystals agree altogether in form with those of 

 leucoplastids ; they are most commonly fusiform, less often rod- 

 shaped. 



Effect of Tension of the Bark on the Formation of the Annual 

 Rings of Wood, and on the Direction of the Medullary Rays.* — 



A careful series of observations on this subject by G. Krabbe leads him 

 to the conclusion that as long as the structure of the bark has under- 

 gone no substantial change either from wounds or from other processes, 

 its tangential tension increases with the increase in thickness of the 

 wood. In opposition to the view of Kny, he considers that the 

 medullary rays are diverted from their original position in conse- 

 quence of the greater contraction of the bark on the side towards 

 which they tend. 



Formation and Properties of Duramen. t — J. Gannersdorfer ap- 

 plies the term duramen (Kernholz) not only to the harder wood 

 towards the centre of stems, but also to the layer of more or less dark- 

 coloured wood often found in the neighbourhood of w^ounds or 

 adjoining dead tissue. The vessels of this duramen frequently furnish 

 thyllge. For the dark-coloured contents of the vessels the author 

 adopts Hartig's term xylochrome. The following are among the 

 more important results of his observations. 



* SB, Akad. Wiss. Berlin, 1882, p. 1093. See Naturforsclier, xvi. (1883;) 

 p. 53, 



t SB. Akad. Wiss. Wien, Ixsxv. (1882) pp. 9-41. 



