BORDERED PITS 



69 







first place, it is to be observed that such positions and modifications 

 of the orifice are invariably associated with the summer wood; 

 if they occur in the spring wood, it is the result of maceration 

 and commonly appears in fossil plants or woods in process of 

 decay, and they are always most conspicuous in those tracheids 

 which have experienced the most profound modifications with 

 respect to the growth in thickness of the secondary walls. It 

 has already been shown in the case of Taxus and Torreya that 

 there is no necessary connection between the spiral bands and 

 the spiral lines of striation, -- that, as a 

 matter of fact, as particularly illustrated 

 by Torreya taxifolia, the two are quite 

 distinct from, one another under ordi- 

 nary conditions of development ; but in 

 cases where the wall experiences extreme 

 growth in thickness the obliteration of 

 the original spiral structure is complete, 

 and at the same time it is replaced by the 

 normal striation of the wall, which then 

 becomes most pronounced. Instances 

 such as those afforded by Pinus strobus 

 and P. insignis may, according to this in- 

 terpretation, be held to represent the final 



phases in the obliteration of the original 



FIG. 15. CUPRESSUS NOOT- 



spirals, and they therefore constitute char- KATENSIS. Radial section 

 acters indicative of the highest type of de- showing deformed bor- 



- , dered pits, x 280 



velopment. In a few cases the structure 



of the bordered pit presents exceptional forms. In Cupressus 

 nootkatensis the pit orifice shows either unusual want of regu- 

 larity in outline and marked eccentricity of position, or it is so 

 enlarged as to leave only a narrow border to the round or oval 

 pit (fig. 15). Similar features occur occasionally in other genera, 

 and they are generally conspicuous in Pinus taeda. De Bary has 

 directed attention to the same feature in Ephedra (13, 159) 

 and Pinus sylvestris, and he correctly interprets it as a form 

 of arrested development. Alterations also arise as a feature of 



