104 VIII. THE MONOCOTYLEDONOUS STEM. 



through. It is this rupture of the partition-wall which enables 

 us to distinguish such elements as vessels or tracheae, in contrast 

 to the pseudo-vascular or tracheidal elements, the partition-walls 

 of which are not broken through. 1 Where trachei'des simulate 

 tracheae in appearance they can be distinguished as vascular 

 trachei'des, in contra-distinction to the fibre-like trachei'des or 

 fibrous trachei'des.- Tracheae and trachei'des are only dead ele- 

 ments, without living contents, which serve for the conduction 

 of water. The two large vessels in the vascular bundle of Zea 

 Mais are either completely surrounded by flat parenchyma cells, 

 or on one side they impinge directly on the elements of the 

 sheath. Towards the sheath elements the vessels bear either no, 

 or small, scattered, pits ; while large pits connect them with the 

 parenchyma cells. Betw r een the two vessels the tissue consists 

 of parenchyma, in which are scattered a smaller or larger number 

 of trachei'des. All these elements are coloured yellow-brown 

 with chlorzinc iodine. 



The part of the vascular bundle which we have thus far con- 

 sidered we distinguish as the vasal or vascular portion, also as the 

 wood or xylem, also Hadrome, The parenchyma of this vascular 

 part we can distinguish from other parenchyma by naming it the 

 vasal parenchyma, or wood parenchyma. The wood parenchyma 

 cells which immediately surround the vessels, which in their 

 form and the structure of their walls betray a relation to the 

 vessels, we can in particular regard as vasal covering-cells. The 

 more or less destroyed vascular elements, which we met with in 

 the inner edge of the vascular bundle, indicate the first elements 

 which were developed in the vasal portion of the vascular bundle 

 and served for the conduction of water during the elongation of 

 this portion while still in the developmental stage. These can 

 be distinguished as the primary wood or protoxylem elements. 



On the outer side of the xylem strand the vasal parenchyma 

 cells are followed by wide cavitied sieve-tubes (v) in regular 

 alternation with their narrow cavitied companion -eel Is (s). 



1 To distinguish microscopically between a true trachea and a vascular 

 tracheide of the spiral or annular type is a matter of extreme difficulty ; 

 hence whether a particular vascular element is or is not a trachea is often in 

 dispute, and for these elements of the spiral and annular type the terms 

 trachea, and vessel (with a reservation as to their exact character) may be 

 in general conveniently used. [ED.] 



2 They might be known as "fibroids". [Eo.J 



