STRUCTURE.] SPIRALS MODIFICATIONS. ? 5 



one another. Their surfaces are studded with small elliptical 

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

 arranged in parallel lines ; these dots are rarely exactly 

 opposite each other, so that the axis of any dot in one row 

 rarely coincides with that of any other in the next. They 

 have no tubular nor rimmed margin. Dr. Griffith further 

 states that, on some of the torn edges, projecting 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 membranous. In the 

 older petioles the tubes are often continuous at their extremi- 

 ties, but in the younger they are not. When these tubes are 

 examined in the dried state, the delicate membrane filling up 

 the dots is said to disappear, 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 microscrope 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 

 perceptible 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, 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. Dr. Griffith 

 finds these tubes always to contain air, except during their 

 earliest periods. Tubes of a similar kind have been figured 

 by Link from ferns (Aspidium, Polypodium, &c.), but they 

 appear to Dr. Griffith to differ from those now described in 

 having a beaded margin and in the dots being opposite each 

 other. These tubes, it is remarked, are not true ducts, 

 inasmuch as they uncoil without breaking, and contain air ; 

 they cannot be considered as any form of woody tissue for 

 the last-mentioned reason, as well as because the dots have 

 a spiral arrangement. They are not scalariform vessels, as 

 their markings do not extend across the tube, nor are they 

 angular. They agree with spiral vessels in, 1 . terminating in 



