the Pitied Tissue of Plant s. 97 



ance of transverse and el!i])t leal pores and dots. The ap])arent 

 pores or dots he believes to be no other than the spaces be- 

 tween the winds of spirals contained within a tube. This 

 theory appears to me totally inadequate to anything like a 

 true explanation of the phcenomena. In all dotted vessels, 

 (excluding the true dotted ducts of Mohl, not alluded to by 

 Dr. Barry,) traces of spiral formation or spiral fibre in some 

 forms may always be met with ; these vessels usually uncoil 

 spirally, and when torn across, the dots appear as spaces be- 

 tween the projecting teeth of the fibres. The fibres, accord- 

 ing to my observations, never run longitudinally to the axis 

 of the containing tube, but always spirally. Therefore, if 

 such be the case, the two interlacing fibres composing the 

 compound spiral must leave spaces whose axes are nearly 

 parallel to the axis of the vessel, and therefore the dots ought 

 to be nearly parallel to the same axis. But that such is not 

 the case, I need not say. The views of Dr. Barry then are 

 not consistent with experience, at least on the formation of 

 dotted tissue. The tissue of which I speak is, I think, gene- 

 rally acknowledged as a modification of true vascular tissue. 

 The dots or pits are formed by spaces left between the fibres 

 but covered by the membrane, so that the wall of the tube 

 opposite the dots is formed by membrane alone ; this may 

 always be found in the younger tissue, but in the older 

 tubes the membrane disappears, leaving them porous. These 

 tubes are very common in Endogenous plants, herbaceous 

 Exogens, and Ferns ; they are often mixed with spiral vessels 

 and sometimes annular ducts. In some of the lower plants 

 {Equieeta, &c.) where these are afterwards found, they appear 

 preceded by annular ducts only, spiral vessels being compa- 

 ratively rare ; whilst in others, as Ferns, annular ducts are very 

 rare, and the remains of the spiral fibre may always be found. 

 In the higher plants we find them in all the stages of deve- 

 lopment. They are more abundant in the older than in the 

 younger plants. The cause of the transformations from spiral 

 vessels, and the regularity of arrangement of the dots of these 

 tubes, depend entirely upon the pressure of the surrounding 

 parts. Mohl has shown that this is the case with the true dotted 

 ducts, and I am convinced that it is the case with the tubes 

 now under consideration ; but I believe that the ai-rangement 

 of Ihe dots of one tube opposite those of the next is not con- 

 stant. When a spiral vessel has formed in a young plant, 

 the rapid growth of the stem induces considerable pressure 

 of the surrounding parts ; the consequence is, that the con- 

 vexity or parts of the surrounding vessels or cells opposite to 

 the spiral \-essel are pressed firmly against it, whilst opposite 

 the intercellular and intervascular spaces the pressure is much 

 Ann. ^ Mag. N. Hist. Vol. xi. H 



