402 Prof. H. Mohl on the Structure of Doited Vessels. 



If we examine the walls of dotted vessels in a direction 

 perpendicular to their surface, we find in almost all plants that 

 both the punctures themselves and the border are extended 

 in length, in a direction transverse to the vessel. In con- 

 sequence the little cavity towards which the canal of the dots 

 leads, and which produces the border, has an elliptic outline. 

 The canal, on the contrary, does not form an elliptic tube of 

 uniform width, but has a form somewhat more complicated. 

 It is compressed in the direction of the axis of the vessel, but 

 at the same time widened in the direction of its diameter. The 

 inner aperture, therefore, of the canal presents a shorter or 

 longer fissure ; the outer, closed by the primitive membrane, 

 presents an ellipse more or less approaching to a circle. If 

 we examine the vessel on its inner surface, and look perpen- 

 dicularly into a canal, we see the lateral portions of its walls 

 running down obliquely towards each other in the shape of two 

 gutter-like surfaces (PL VII. fig. 4, Cassyta glabella) ; while 

 its upper and lower walls are invisible, being perpendicular to 

 the eye of the observer. From this form of the canals it is 

 clear why they appear under a different aspect according as a 

 section is made vertically or transversely ; in the former case 

 (PL VII. fig. 9, Laurus nobilis) they exhibit a conical, in the 

 latter (PL VII. fig. 2, Cassyta glabella) a cylindrical outline. 

 Moreover a transverse section exhibits a very different form, 

 according as it is made near the outer or inner aperture. In 

 the former case it has a broad elliptic form ; in the latter it 

 has more resemblance to a linear fissure. This is clearly vi- 

 sible if a longitudinal section be made in an oblique direction 

 through the wall of a vessel. 



The extension in width which the canals of the dots exhi- 

 bit within, is in a portion of Dicotyledons not very remark- 

 able, so that the inner aperture is shorter than the border of 

 the dot; e.g. in Cassyta glabella (PL VII. fig. 1, 4), Bombax 

 pentandrum (PL VII. fig. 12), Bixa Orellana, Acacia lophantha, 

 Sophora japonica, Salix alba, Aralia spinosa; in other plants, 

 on the contrary, the inner aperture presents a fissure which 

 is longer than the border, e.g. in Laurus Sassafras (PL VII. 

 fig. 5), Aleurites triloba (PL VII. fig. 6, 8), Clematis Vitalba 

 (PL. VII I. fig. 4), Cornus alba, Morus alba, Gynmocladus cana- 

 densis, Elceagnus acwninata (PL VII. fig. 10, 11). In this case 

 it very frequently happens, and in many vessels with a cer- 

 tain degree of regularity, that the fissures of neighbouring 

 dots run together, so that the inner wall of the vessel is 

 pierced with transverse or obliquely situated grooves, into 

 which from two to six and often more canals open. Exactly the 

 same structure as in the elliptic dots is found in the transverse 



