ANOMALOUS THICKENING IN DICOTYLEDONS AND GVMNOSPERMS. 599 



latter case only will be here taken into account. Opposite each face of the vascular 

 plate a strand of wood and bast containing much parenchyma is formed in the normal 

 way, and before each angle a broad primary medullary ray. In the pericambium a 

 periderm begins simultaneously to be formed, and by it the outer cortex is thrown off. 

 The primary medullary ray consists exclusively, and the bast mainly, of radially seriate 

 parenchymatous cells, which gradually become greatly elongated in the same direction : 

 these together form a thick layer inside the periderm. In the peripheral portions of this 

 layer, near to the phellogenetic zone of the periderm, tangential division begins all round, 

 at the time when the root begins to swell strongly, and thereby a fresh (renewed) 

 cambium is started. The formative activity of the first normal layer now falls off, and 

 soon ceases altogether. The second cambium alone continues the secondary thicken- 

 ing of the root throughout life. By means of tangential divisions proceeding for the most 

 part centrifugally, it forms alternately radially arranged parenchyma and vascular bundles. 

 The latter are connected in radial and tangential direction in acute-angled meshes, and 

 are arranged in the transverse section in tolerably regular annular zones, which alternate 

 with zones having no bundles, and differ from the latter also by the less marked radial 

 extension of the parenchyma ; they are thus very conspicuous. 



Fundamentally the same phenomena of thickening appear in the hypocotyledonary 

 portion of the stem, which takes part in the turnip-shaped thickening of the main root: 

 the peculiarities of this part are irrelevant, and may pass unnoticed here'. 



3. In the root of the cultivated forms of Beta"^, a diarch primary vascular bundle was 

 always found in the cases investigated : the secondary thickening begins fundamentally 

 in the same way as in Mirabilis, and proceeds for a time in a normal manner. The 

 cortical parenchyma surrounding the outer margin of the primary groups of bast thus 

 becomes much thicker opposite the two secondary bundles than opposite the angles of 

 the primary vascular plate; I will not say for certain whether this is the result, as stated 

 by van Tieghem, of a formation of phelloderm, starting from the pe -icambium which 

 acts as a phellogenetic layer, or of active growth of the parenchymatous elements of the 

 primary bast (i.e. of the primary phloem strands) seated within the pericambium. 

 After a time, in the case of the main root of B. vulgaris when it is about 5™" thick, the 

 formation of a new cambial zone starts in this peripheral layer of parenchyma on both 

 s ides, by regular successive tangential divisions appearing in an annular layer of cells. 

 S ince this process is continued, from the middle of the two points indicated, laterally 

 towards and over the two main medullary rays, a completely closed cambial zone is 

 formed, which undergoes active reciprocal tangential division : in it are differentiated 

 strands of wood, and of bast corresponding to them, and medullary rays, which are 

 further developed after the manner of normal secondary thickening. Later, the growth 

 of this second thickening zone ceases, and it is replaced by a new one similar to it, 

 which had already begun to appear at the outer limit of the chiefly parenchymatous 

 second layer of bast, in the form of tangential divisions, which arise at scattered points, 

 and extend thence laterally all round. Since the same process is repeated, there arise 

 in the Beet those well-known concentric rings of wood, broken up by medullary rays, 

 which alternate regularly with the chiefly parenchymatous zones of bast, the number of 

 which may be as many as six and more in a strong one-year-old beet. According as the 

 rings are further from the centre, and therefore wider, the number of their strands of 

 wood and bast increases. 



Between the bundles of successive rings there are obliquely descending radial con- 

 necting-bundles. Further, instead of a ring, larger or smaller segments of a ring may 

 appear here and there, which are then connected by their margins to the next inner 

 rings. Here also the hypocotyledonary axis takes part in the formation of the beet, the 

 phenomena being similar to those in the main root. '" 



^ Compare van Tieghem, /. c. 

 [See Droysen, Beitr. z. Anat. u. Entw. d. Ziickeiiube, Halle, 1877.] 



