STEM OF ZEA MAIS. 105 



They serve for the conduction of proteids. This string of tissue 

 is accompanied on the flanks by a layer of thin-walled cells ; still 

 farther outwards it is succeeded by a narrow layer of strongly 

 swollen functionless sieve-tubes and companion-cells (pr)> These 

 last have taken a brownish colour in the chlorzinc iodine, while 

 the other elements are violet. The companion-cells in the active 

 portion are distinguished also by rich protoplasmic contents, 

 stained yellow-brown. Here and there the cross-section may 

 have skirted the end wall of a sieve-tube, which will then show 

 the finely punctate sieve-plate. 



To distinguish it from the vasal part, this portion of the vas- 

 cular bundle, composed of sieve-tubes and companion-cells, is 

 called the sieve portion ; also by the names Bast or Phloem, and 

 likewise Leptome. For the entire vascular bundle, composed of 

 xylem and phloem portions, the term vascular bundle may be 

 used, as also in some cases fibro-vasal or fibro-vascular bundle. 

 In contrast to Hadrome and Leptome the collective term Mes- 

 tome may also be used. 1 The swollen elements situated in the 

 periphery of the sieve portion belong to the time of the first 

 development of the bundle, when they were themselves newly 

 developed, and active, and (just as with the protoxylem) they 

 constitute the protophloem, the first formed elements of the bast. 



Such vascular bundles as this, in which the sieve portion 

 impinges unilaterally upon the vasal portion, are known as col- 

 lateral. As in the development of the bundle no tissue capable 

 of division persists between the vasal portion and the sieve 

 portion, such bundles are spoken of as closed. 



The layer of thin- walled unlignified cells along the flanks of 

 the bast appertains to the vascular bundle sheath ; and in the 

 same way the protophloem is bounded by sheath elements, but in 

 this case thickened and lignified. These last are sclerenchyma 

 fibres, but are distinguished by their exceptional width. The 

 sclerenchymatous elements of which the sheath is composed 

 pass over by a few intermediate elements (belonging, doubtless, 

 to the ground-tissue, but the position of which has prevented 

 expansion) into the large-celled parenchymatous ground-tissue 

 (the conjunctive-tissue) of the central cylinder (or stele). The 



1 The terms " Vascular portion " and " Sieve portion " were given by De 

 Bary ; Xylem and Phloem 'by Nageli ; Hadrome, Leptome and Mestome by 

 Haberlandt and Schwendener. These last-named terms have a wholly 

 physiological significance. [ED.] 



