THE VEGETABLE TELL. 



19 



wist of the substance capable of swelling tip, e. g., in the hairs of 

 the seed of Mu^ilkt strepitans. 



Ohserv. 1. Hartig, who first discovered that the tertiary membrane in 

 Taxiis possessed the form of a comiected pellicle and was not composed of 

 fibres, propounded the doctrine ("'ilei^/W//^ ^ii7^ E Mwlchluwjsgesch. derPjlmb- 

 zen^^ 1843) : that such an inner coat, which he called tha 2>tyQ!tode^ occurred 

 in all cells. Ho thought that this membrane was distinguishable from 

 the intermediate layer (his Astathe) by definite chemical characters, since 

 it was not coloured blue by iodine and sidphuric acid, like the latter, and 

 agreed in this character with the outermost coat of the cell (which he 

 called the Eustathe). Hartig considered this inner layer as the oldest, the 

 outermost the youngest, of the cell-membranes. The whole of this doe* 

 trine depends upon very imperfect observations. The tertiary membrane 

 of TdxiM is composed of cellulose, it is therefore a true cell-mcm]>rane ; but 

 Hartig seems, in many other cases, to have taken the primordifd ntnch 

 (subsequently to be described) for a layer of cell-membrane, and thus to 

 have classed together structm^es which have notliing at ail in common. 



Ohsevi), 2. It may not be out of place, after this exposition of the struc- 

 ture of the secondary membranes, to cast a glance at the structure of the 

 vascular utricle, since the different modifications of the structure of the 

 cell-wall are met with again in the vessels, and, indeed, in many cases dis- 

 played much more distinctly than in the cells, so that these conditions 

 were observed in the vessels long before they were known in the cells, 

 albeit much that was incorrect was stated of them. The vessels were 

 divided according to the modifications of the structure of their secondary 

 layers, into spiral, annular, reticulated, dotted vessels, &c. 



The most widely distributed form is the spiral mssel, for this occurs in 



all plants which 

 Ftg, 27, Fiff. 28. Fiff. 20. possess vessels ; and 



I { particularly, in most 

 organs the first ves- 

 sels which appear 

 belong to this form, 

 so that they are met 

 with in the hind- 

 most parts next the 

 pith, of the vascu- 

 lar bundles of the 

 stem. The secon- 

 dary membrane of 

 these vessels is di- 

 vided into one or 

 more (in Mvi^a as 

 many as 20) paral- 

 lel spiral fibres, 

 which as a rule ter- 

 minate in an annu- 

 lar fibre attheupper 

 and lower ends of 

 the vascular utricle. 

 If the vessel is developed in an organ which has already completed its 



c 2 



Fig, 26. 



Spiral vessel of Jmjpcc- 



Spiral vessels of SamhiMcus 

 Mhulm. 



