214 R. BRAITHWAITE ON TFIE HISTOLOGY OF PLANTS. 



striation of the cell wall. In Vinca and some other Apocynese both 

 left and right-handed complex sph-als occur, 



3. Beticular thickening layers are common in parenchymatous 

 cells of the leaves in Liliaceae, in capsules of Hepatic£e, in the 

 collenchyma of elder, and in the bark tissue of Balsam. 



4. Porose thickening layers are seen in most cells of the higher 

 plants, the pores appearing as small flat rings where the secondary 

 deposit is very thin, but as a cylindric canal when this layer is 

 thick. We have closed and open pores, though originally all are 

 closed ; but where pores of adjoining cells correspond, absorption 

 of the^intervening cellulose case converts them into a canal, by 

 which air passes into the interior. Closed pores are especially seen 

 in the parenchyma, or in all cells destined to prepare and store up 

 reserve material. The pore canal is usually cylindric, but some- 

 times it is narrowed externally, and in very strongly thickened 

 parenchymatous cells the pore canals are often branched, as we see 

 in the thickened woody parenchymatous cells forming the hard con- 

 cretions in winter pears, in the shells of stone-fruits, in the albumen 

 of the ivory-nut. and in the bark of Ash, Fir, Hoya, &c. Closed 

 pores may be also somewhat widened at base, so that the real 

 aperture appears bordered by a second circle. Similar bordered 

 open pores are found in all cells, which having finished their 

 thickening layers only carry air; these are seen in most wood cells, 

 in all bast cells, and also in all vascular cells of woody tissues. In 

 bast cells we often find that the aperture of the canal becomes so 

 small that the border is indistinct. 



In the vascular cells the bordered open pores appear externally 

 on the lateral walls, and also on the transverse ones, and thus free 

 communication is established throughout, as we see in Ash, 

 Maple, &c. 



If the border as well as the pore canal be extended considerably 

 in width, we have the form known as Scalariform, or ladder-like, 

 so very common in the vascular cells of Ferns, Lycopods and 

 Monocotyledons, and also in the partition walls of the pith in 

 Conifers. Combined porose and spiral thickening occurs in some 

 wood cells as in those of Yew, Viburnum Lantana, &c., and in 

 vascular cells of Maple, Lime, and Honeysuckle, the spiral band 

 being a tertiary layer. Other peculiar forms are seen by transverse 

 section, in the parenchyma of the leaf of Cycas reuoluta as semi- 

 cylindric bands, and of Finus sylvestris, where they project like 



