PITTED STRUCTURES. 



[ 510 ] 



PITTED STRUCTURES. 



}3its, however, which lie on parts of the 

 wall adjoining the cells of medullary rays, 

 are generally devoid of the rim. 



Similar bordered pits occm* very generally 

 on the walls of the pitted ducts of Dicotyle- 

 dons ; but as the wood is here of mixed com- 

 position, and the ducts adjoin cells as well as 

 other ducts, independently of the medullary 

 rays, we often find a greater variety of con- 

 ditions on the wall of the same duct, which 

 may have bordered pits when adjoining an- 

 other duct, and simple pits, or pits wdth a 

 double outline, when adjoining cells. The 

 pits \^ith a double outline (PI. 39. figs. \5 b, 

 & 20) are of diiferent nature from the bor- 

 dered pits (PL 39. figs. 13, 14, 15 a, 16, 18), 

 the double outline depending simply on the 

 fact that the later or more internal layers of 

 thickening do not reach the edge of the ori- 

 fice in the earlier secondary deposits, so that 

 the pit is conical, or rather, has sloping 

 edges, the circumference at the primary 

 membrane being rather less than that of the 

 margin next the cell cavity. A peculiar 

 modification of this unequal mode of deposit 

 is seen in company with the true rim or 

 border in many cases (PI. 39. figs. 14. 16. 

 18), where the central spot or original pit 

 appears in the middle of a slit running across 

 the circle indicating the border; this slit 

 indicates the alteration of the shape of the 

 gap in the secondary deposits in the success- 

 ive layers, and corresponds to the inner 

 margin of the pit, where this has the form of 

 an elongated groove or slit, gradually dimi- 

 nishing to a small round hole towards the 

 primary cell-membrane (PI. 39. fig. 18 a). 

 Sometimes (PI. 39. fig. 18 a, b) the two or 

 more slits formed in this way on contiguous 

 pits become confluent. The last condition 

 indicates a transition to the more sparing 

 form of the secondary deposit where it ap- 

 pears as a modification of a spiral fibre or 

 fibres ; and the later secondary deposits of 

 pitted ducts do sometimes actually assume 

 this form, and produce a spiral fibrous layer 

 of thickening inside the layers perforated by 

 pits. This is the case in Taxus (PI. 39. 

 fig. 4), in the Lime (PI. 39. fig. 13), and 

 Mezereon (PI. 39. fig. 19 b), &c. 



For the guidance of microscopic observers 

 we may furnish a series of examples in addi- 

 tion to the CoNiFERvE (PI. 39. figs. 1. 4. 6) 

 of different kinds of marking on pitted ducts. 



A. Forms where there is no spiral-fibrous 

 secondary deposit. 



a. Ducts with bordered pits uniformly 



distributed, without reference to adjacent 

 structures : Eleagnus acuminatus. Clematis 

 Vitalba (PI. 39. fig. 18). 



b. Ducts where the bordered pits are 

 fewer on the walls adjoining cells : Acacia 

 lophantha, Sophora japonica. 



c. Ducts with bordered pits where adjoin- 

 ing ducts, while the walls adjoining wood- 

 cells have few or no bordered pits, and 

 those next the medullary rays have pits 

 without a border : elder, beech, hazel, pop- 

 lar, alder, plane, apple, &c. 



d. Ducts with bordered pits where adjoin- 

 ing ducts, but with large pits devoid of a 

 border where adjoining cells : Cassyta gla- 

 bella (PI. 39. fig. 14), Bombax pentandra 

 (PI. 39. fig. 15). 



e. Ducts presenting a modification of the 

 last, where the bordered pits have the form 

 of slits as wide as the ducts when adjoining 

 ducts, while the walls adjoining cells have 

 large pits without a border : Chilian thus 

 arboreus (PI. 39. fig. 17), the vine (in a less 

 striking manner). Eryngium moritimum 

 (PL 39. fig. 21) exhibits a condition ap- 

 proaching this. 



B. Forms where a spiral-fibrous structure is 

 added after the pits. 



f. All the ducts with bordered pits, but 

 the larger ducts with smooth walls, the 

 smaller with a spiral fibre : Clematis Vitalba, 

 Ulmus campestris, Morus alba. 



g. All the ducts closely pitted, with slen- 

 der fibres between the rows of pits : Hakea 

 oleifolia. 



h. The larger ducts with pits, the smaller 

 without ; both kinds with spiral fibres on the 

 internal surface : Daphne Mezereum (PL 39. 

 fig. 19), Passerina filiformis, Genista cana- 

 riensis. 



i. The walls adjacent to other ducts pitted, 

 those next cells with very distant pits, or 

 devoid of them ; all the walls with fibres : 

 the lime, horse-chestnut, sycamore, cornel, 

 holly, hawthorn, Prunus Padus, virginiana, 

 &c. 



Tlie last set of forms allies these struc- 

 tures to those characterized peculiarly by 

 the Spiral-fibrous structures, and, as 

 will be indicated there and under Second- 

 ary DEPOSITS, the smooth lavers of thick- 

 ening, such as those between the pits of 

 Pinus, may be made to show a spiral struc- 

 ture by the action of reagents. 



For the micro-chemical conditions of these 

 objects, their development, and relations. 



