Prof. H. Mohl Oil I fie Structure of Dotted Vessels. 397 



in these tubes, those portions only of the lateral walls which 

 abut on other tubes are furnished with large dots surrounded 

 by a border, while in the portions abutting on the medullary 

 rays in most species of Pinus, Juniperus, &c., are many small 

 dots destitute of a border, which agree altogether with those 

 which appear on the cells of the medullary rays themselves. 

 Not only the dependence of their formation upon the conti- 

 guous organs appears from these tubes, but also more espe- 

 cially from the circumstance that the organization peculiar to 

 the dotted tubes takes place only in such parts as are not sub- 

 jected to that extraneous influence proceeding from cells. 



It will scarcely be objected successfully to the above po- 

 sition, that the influence of contiguous cells cannot be proved 

 in the dotted tubes of all plants, since in a portion of them 

 the dotted tubes exhibit uniform dots in every part, whether 

 in contact w ith cells or vessels. This clearly proves only thus 

 much, that the influence which neighbouring cells exercise 

 on the formation of vessels is not so important under all cir- 

 cumstances as to prevent the formation of that species of dots 

 peculiar to porous tubes, but that in some plants the power 

 of organization peculiar to the vessels is so exceedingly strong, 

 that, notwithstanding the influence of the neighbouring cells, 

 the peculiar structure of the dotted tubes is eflected more or 

 less completely. But from such exceptions an inference can 

 by no means be deduced that this influence does not exist at 

 all*. 



* If we consider generally the dejiendence of the organization of the se- 

 condary coats of one elementary organ on tliat of neighbouring elementary 

 organs, it appears that in this respect there are many degrees of intensity, 

 and that the circumstance is of especial importance, whether the secondary 

 coats of an organ exhibit or not a more or less evident spiral structure. 

 When, for instance, in an elementary organ, whether it be a cell or vascular 

 tube, the secondary coats exhibit a very decided and regular spiral structure, 

 we can find no trace of any influence of the neighbouring organs on the 

 formation of these spiral secondary coats. We observe, then, dextral and 

 sinistral, closely and distantly wound spiral vessels, containing one or more 

 threads, lying near each other; and in like manner, spiral cells similarly 

 circumstanced when their threads are well-developed. In either case the 

 independence of the neighbouring organs appears not only from their tlu'eads 

 running in different directions, but especially from the circumstance that 

 the intervals between the threads run uninterruptedly over the angles of their 

 own elementary organ, and over the places where the walls of neighbouring 

 organs stand pei-pendicular to the outer surface of the former. If, on the 

 contrary, the spiral structure is less decided, and the secondary coats do not 

 any longer assume the form of isolated threads, but exhibit only a spiral 

 streak, tliere commences immediately a dependence of one elementary organ 

 on its neighbour. If under such circumstances dots are formed, they corre- 

 spond exactly in their position, but only imperfectly in form, since in cither 

 elementary organ lying on the other they may be elongated in the direction 



