FIBRO VASCULAR TISSUES: VESSELS 99 



simulates the vessel of Pteris aquilina figured in connection with 

 an earlier paragraph. Although the result is the same, the manner ' 

 of reaching it is different in the two cases, and a distinction should 

 of course be made in drawing any evolutionary conclusions. It 

 will be clear to the reader from the evidence presented in the 

 present and in a former paragraph that the vascular structures of 

 the lower dicotyledons do not originate as in the Gnetales simply 

 by enlargements of pits and the disappearance of the pit membranes 

 in the terminal regions of the vessels. On the contrary, they 

 typically take their origin as a consequence of the lateral fusion 

 of horizontal rows of pits with a correlated disappearance of the 

 membranes. The vessel in the lower angiosperms is, however, as 

 clearly a derivative of the tracheid as it is in the case of the highest 

 of the gymnosperms. Further, in the angiosperms the vessel or 

 trachea, as a result of its much more complex mode of evolution, is 

 more distinct from the fibrous or tracheary element than it is in 

 any of the lower groups. 



The type of vessel characteristic of the higher dicotyledons and 

 the mass of monocotyledons may now profitably occupy our 

 attention. Fig. 77 illustrates three vessels belonging to the 

 category of elements with porous end walls (that is, vessels with 

 so-called porous perforations). In a is shown such an element 

 from the oak in approximately radial aspect. The terminations 

 of the vessel taper and are distinguished by the large aperture or 

 pore. The lateral surfaces are covered with pits, which are of two 

 kinds. The smaller ones, provided with a distinct border, put the 

 vessel in relation with other similar structures and with tracheids. 

 The simple and slightly irregular pits represented in about the 

 middle horizontal region indicate the presence of a small ray in 

 contact with the vascular element. In b is represented a vessel 

 from the wood of the poplar in radial view. The same slanting 

 ends and large terminal pores as are present in the oak similarly 

 characterize the vessel of Populus. The uniform crowded lateral 

 pitting indicates that the face of the vessel presented to the observer 

 was in contact with another vessel. In c is presented the somewhat 

 tangential view of a vessel in the maple. The ends of the vascular 

 element do not differ essentially from those of the oak and poplar. 



