76 V. CELL-WALL, AND CELL CONTENTS. 



the pit, as commonly is the case in the narrower cells, is an 

 obliquely placed ellipse (as in A), by changing the focussing 

 we shall find the corresponding mouth of the other pit oblique 

 in the opposite direction. The two pit-chambers adjoining one 

 another are separated from one another by the primary wall, 

 which, before the commencement of the secondary thickening, 

 was already present, and is only slightly thickened subse- 

 quently. This delicate wall is the closing membrane. In the 

 middle it is somewhat thickened, and forms the so-called torus, 

 which is characteristic of the closing membrane of bordered pits. 

 With most careful observation and suitable focussing with a 

 sufficiently high power we may even be able to see this torus. 

 It forms a round, weakly-shining disk, which has about twice the 

 diameter of the mouth of the pit (compare in A). In the most 

 favourable cases, and especially in preparations of dried wood, 

 radial striation is observable in this torus, so that the delicate 

 part of the closing membrane appears differentiated into radially 

 arranged lamellae (Fig. 27, A). 



A complete insight into the structure of the bordered pit can 

 only be obtained with the aid of tangential sections. As the 

 bordered pits stand on the radial walls of the wood-cells, they 

 are seen in cross view (Fig. 27, B) in correctly taken tangential 

 longitudinal sections. We search for these structures in the 

 walls separating the wood-cells, stopping first at the dividing 

 walls of the broader wood-cells, and not allowing ourselves to be 

 led astray by the sectional view of the medullary rays, which are 

 formed of a number of smaller cells standing one over the other. 

 The figure of the cut pit is, it is true, clear only in very delicate 

 parts of the section. If this condition is fulfilled, the pit appears 

 in the form of the two pairs of pincers directed towards one 

 another (or like a couple of extremely short screws placed with 

 their heads flat together), much as in the above figure (27, C). 

 If once the structure of these large-bordered pits is known, we 

 can easily realise the structure of the smaller ones, which lie in the 

 thicker walls of the narrower wood-cells. The difference, apart 

 from the smaller size, is, that here on both sides a longer canal, 

 corresponding to the thickness of the wall, runs out of the 

 broadened pit-chamber. Between the largest-bordered pits and 

 the smallest are all intermediate stages. In the interior of the 

 pit is seen, in the most favourable cases, the closing membrane, 

 which in its centre is swollen into a torus (f). In the bordered 



