THE CELL-WALL. 



489 



The annular thickenings (fig. 539) are less common than the 

 spiral, but occur sometimes in the same cell, and also in association 

 with the next kind, the reticulated. The rings are generally at 

 some little distance apart. 



The reticulated secondary layers may be uniform over the wall o 

 the cell, or irregular (tig. 540), which' is more frequent, since the 

 ordinary cause of the reticulated appearance is the formation of 

 vertical connecting bars between rings or spiral coils at the angles 

 of the cells ; when this occurs very regularly, a ladder-like arrange- 

 ment results, giving what is called the scalariform structure, espe- 

 cially frequent in the vascular structure of Ferns (fig. 541). 



Fig. 541. 



Fig. 542. 



Fig. 541. Fragment of a scalariform vessel of a Tree Fern : a, walls in contact with other 



vessels : b, b, walls in contact with cells. Magn. 200 diam. 

 Fig. 542. Wood-cells of Yew ; vertical beetion. Magn. 300 diam. 



The connecting bars of the reticulated and scalariform cells must not 

 be supposed to originate after the rings or spirals ; they are contempo- 

 raneously developed ; and the diversities in the closeness of the coils of 

 cells are likewise original peculiarities of the deposits. The statement 

 that the turns of spiral coils are opened by longitudinal growth of the 

 primary membrane to which they adhere seems to be founded on specu- 

 lative notions. 



The spiral structure of secondary deposits is beautifully seen in the 

 elaters of Junf/ermannia and Marchantia, in the cells of the aerial roots of 

 epiphytic Orchids, in the cells of the wood of Cactaceae, and in the spiral 

 vessels of the veins of the leaves and leaf-stalks of Monocotyledonous 

 plants, such as the Hyacinth, Narcissus, Musa (which presents as many 

 as 20 parallel bands), shoots of Elder, leaf -stalks of garden Rhubarb, 

 Strawberries, &c., also in the petals of delicately organized flowers. 

 Annular cells are well seen in the sporanges of Marchantia and other 

 Liverworts, and in many of the structures just mentioned with spiral and 

 reticulated cells. 



Scalariform Tissue. The scalariform marking is most regular in 

 Ferns, and approaches very nearly to the more regular forms of the 

 thickening above described, so that the spiral-fibrous and tjie dotted 

 forms appear as the extremes of an analogous kind of structure. 



