96 



joint of the arms, and as the plant elongates with an indefinite num- 

 ber of knots, it must be very produetive of seeds. 



In the first place I have to remark that the number of arms, h, h, at 

 each knot, has a remarkable connexion with the tubes covering the 

 stem, see fig. 8, and these tubes are of two sorts, primary and secon- 

 dary. Under each arm, h, one primary tube, d, descends, and one 

 also ascends above each arm : these last meet other primary tubes 

 descending, set midway on the stem, and each of these divides into 

 two secondary tubes, so that with nine arms there will be nine pri- 

 mary tubes and eighteen secondary ones, making twenty-seven around 

 the stem, while with seven arms there will be only twenty-one. 



Secondly. The primary tubes are formed of several lengths, a and 

 b y ft, b, joined together in one line by very peculiar cells, c, d, fig. 

 10, and these primary tubes are joined to the plant by their first cells 

 only, a ; and the secondary tubes, e, f, are still more detached, their 

 sole connexion with the primaries being by means of their small cells 

 c, d y from which they emanate, like side-leaves from a central stalk. 

 One of the ascending and two of the descending sets are pulled 

 away to show their structure. 



Thirdly. The ascending and descending primary tubes never come 

 into contact with each other, but reciprocally end between the oppo- 

 site secondary tubes. 



Fig. 8 shows this alternate junction midway on the stem, and fig. 

 9 the general appearance of this alternate meeting on an extremely 

 young stem. 



Fourthly. All the covering-tubes grow close together, with a slight 

 lateral adhesion, and so form a coat to the stem, but they are pre- 

 vented from touching it by a layer of carbonate of lime, which 

 crystallizes round the stem beneath the coating. 



Fifthly. The primary tubes are much larger than the secondary 

 ones and thus give an angular form to the stem, as shown in the sec- 

 tion, fig. 11 : this allows more room for the coating of carbonate of 

 lime beneath the secondary tubes, and therefore it is most frequently 

 to be seen through them, as indicated in fig. 8. 



Sixthly. The covering-tubes sometimes so much outgrow the 

 stem as to form a very loose jacket, like fig. 12, their extreme ends, 

 a, both at top and bottom, continuing united to the plant. 



In other cases the stem out-grows the covering tubes : and then 

 bends out, as in fig. 18, in which case it resembles a bow, the tubes 

 forming the string. This shows the tubes to be detached from the 



