ZOOLOGY AND BOTANTj MICROSCOPY, ETC. 259 



(4) Structtire of Tissues. 

 Molecular Structure of Vegetable Tissues.* — From an esammation of 

 the polarizing piienomeiia of the cells and tissues of a large number of 

 plants belonging to Cellular Cryptogams, Vascular Cryptogams, and Phan- 

 erogams, Herr N. J. C. Miiller classes them under four types as respects 

 molecular structare. In the first two types the molecules have a globular 

 form, with the radial axis either longer or shorter than the two tangential 

 axes, which are equal in length. In the two other types the molecules are 

 cylindrical, with optical axes of three different lengths, one radial, another 

 parallel to the axis of the cylinder, the third also tangential, but at right 

 angles to the axis of the cylinder. Of these the longitudinal axis is always 

 either the longest or shortest, the tranverse axis being always intermediate 

 in length between the two others. The relative position of the axes is not 

 unfrequently disturbed by torsion. 



Nuclear Sheath.t — Dr. H. de Vries traces the presence of a nuclear 

 sheath right up to the growing apices of roots ; it is this layer, and not the 

 pericambium, which limits the pressure, as is seen from the phenomena in 

 older roots. These two layers form together a stratum of close cells with- 

 out intercellular spaces. The currents of protoplasm in the various layers 

 of tissue are described, and especially in the nuclear sheath. The granules 

 pass here in a broad stream along the tangential and transverse walls. 

 There is also a constant current of protoplasm in the living cells of all the 

 layers of tissue in the young roots, and here also especially along the 

 tangential and transverse walls. The direction of the current is such as to 

 serve for the transport of water from the root-hairs to the vascular bundles, 

 and of nutrient substances from the older parts of the root to the layers of 

 growing tissue. 



Annual Formation of Cork. J — According to Herr A. Gerber, it is not 

 in all trees and not in all seasons that the cork forms a distinct ring every 

 year. He distinguishes three distinct types in this respect. The yearly 

 increase of cork-varies greatly, from one row of cells in Salix to 100 rows in 

 Quercus suher. It is usually strongest in the first year, and nearly constant 

 after that. The number of rows in cells stands in inverse proportion to 

 the thickness of their walls. 



Pericycle.§ — M. J. d'Arbaumont expresses views somewhat divergent 

 from those of Van Tieghem || as to the origin of this tissue. He regards the 

 central cylinder as divisible into two main parts or regions, one corre- 

 sponding to the primordial conjunctive tissue comprising the pith and the 

 whole or a portion of the primary medullary rays, the other to the 

 secondary formative tissue, from which proceed, on the one hand the xylem, 

 on the other hand the soft bast or pericycle. 



In fi.ve herbaceous plants examined, the author found the pericycle to 

 be a product of differentiation of an unbroken zone of formative tissue, 

 independent of the primordial meristem, from which the soft bast and xylem 

 proceed ; this zone is an integral portion of the fibrovascular bundles. In 

 the Cucurbitacese we find a different structure ; the pericyclical layer is 

 divided into two parts, one internal, parenchymatous, broken up, and 

 remaining adherent to the xylem of the bundles, the other external, fibrous, 



* Pringsheim's Jahrb. f. Wiss. Bot., xvii. (1886) pp. 1-49 (4 pis.). 



t Maandbl. v. Natuurwet., xiii. (1886) pp. 53-68. See Bot. Ztg., xliv. (1886) p. 788. 



t Zeitschr. d. Naturwiss., iv. (1886) pp. 451-88. 



§ Bull. Soc. Bot. France, viii. (1886) pp. 141-51. 



II See this Journal, 1886, p. 266. 



