SPIRAL TRACHEIDS 39 



structure to project from all sides and extend over the area <>1 

 arrested growth as a lip which never completely closes at the 

 center, where there is left a usually circular, sometimes oval or 

 again lenticular or even oblong, opening, and in this manner the 

 bordered pit is formed (plate 6). 



From the statements so far presented it may be correctly 

 inferred that the structural alterations which arise within the 

 transition zone are subject to great variations, whereby the 

 change from spirals to bordered pits may arise very gradually 

 through a broad, radial zone, as in Cordaites Brandlingii, or it 

 may take place very abruptly, as in the modern Coniferae. The 

 general tendency of such evidence is to show that with a higher 

 type of organization there is a corresponding diminution in the 

 transition zone and an increased abruptness in the structural 

 alterations. The logical result of an extension of this process 

 would be the reduction of the bordered pit to the condition of 

 a simple pit, and ultimately to its complete obliteration. In the 

 Coniferae the reduction of the bordered pit to a simple pit some- 

 times occurs in the case of medullary rays or even in the case 

 of tracheids with very thick walls, but it becomes most promi- 

 nent in the Angiosperms, where it is a characteristic feature. 

 Instances also occur in some of the hard pines, in which the pit 

 is completely obliterated. This applies in particular to tracheids 

 of the summer wood, the walls of which have become unusually 

 thickened. 



The relations to which attention has thus been directed some- 

 what in detail have been expressed in more general terms by 

 De Bary (13, 321) in the statement that, "Outside the primi- 

 tive elements wider tracheae follow. Their development takes 

 place successively, advancing from the inner edge of the bundle 

 outwards, and, as a rule, at a time when the elongation of the 

 entire part to which they belong is nearly at an end. The 

 thickenings on their walls, therefore, have a successively denser 

 arrangement ; dense spirals and annular tracheae, then reticu- 

 lated and pitted tracheae, follow one another in succession from 

 within outwards, with gradual transitions, or with the omission of 



