Dr. Griffith on the Dotted Vessels of Ferns. 169 



XXV. — On the Dotted Vessels of Ferns. By J. W. Griffith, 

 M.D., F.L.S. 



[With a Plate.] 



During the examination of the petioles of some of the Bri- 

 tish Ferns, I have several times been led to doubt whether 

 the dotted tubes found in them * have really been referred to 

 their proper situation in structural classifications, and if their 

 real functions have been properly understood. They are 

 usually considered as forms of woody tissue. 



These tubes are situated in bundles at tolerably regular 

 distances from the axis and from each other, surrounded by 

 the cellular system of the petiole. In the younger petioles 

 they are mixed with spiral vessels, but these are rarely found 

 in the older ones. Their transverse section shows them to be 

 cylindrical or elliptical, not angular nor solid (PL V. fig. 1. a.). 

 They are usually of a yellowish brown colour, terminating in 

 acute extremities, which become more obtuse as their age 

 advances. In situ their terminations overlap one another 

 (fig. 3. a.). Their surfaces are studded with small elliptical 

 markings or dots, not extending far across the tube, but ar- 

 ranged in parallel lines ; these dots are rarely exactly opposite 

 each other, so that the axis of any dot in one row rarely coin- 

 cides with that of any other in the next. They have no tu- 

 bular nor rimmed margin. On some of the torn edges pro- 

 jecting solid fibres may be seen leaving spaces between them 

 corresponding to the dotted parts, and sometimes on their 

 edges may be seen the fragments of the lacerated membrane 

 filling up the dots, thus proving that these tubes are composed 

 of two coats, one of united fibres, the other delicate and mem- 

 branous. In the older petioles the tubes are often continuous 

 at their extremities, but in the younger they are not. When 

 these tubes are examined in the dried state the delicate mem- 

 brane filling up the dot disappears, leaving a perfect foramen. 

 The dots are situated obliquely on the walls of the tubes, so 

 that if the upper and under surfaces be brought into focus 

 under the microscope immediately after one another, or the 

 focus of the object-glass be made to correspond to the centre 

 of the tube so as to have both surfaces indistinct but still per- 

 ceptible at the same time, the dots cross one another, showing 

 their arrangement to be spiral. When they are stretched they 

 do not break but uncoil (PL V. fig. 4. «.), as if the tube were 

 formed by a band of four or five spiral fibres united at the 

 margins. Their terminal points are situated on one side so as 

 to make the end appear cut off obliquely. Sometimes there 

 * Pteris aquilina shows them remarkably well. 



