R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 213 



of these arrests causing an empty interspace, which is indicated by 

 a dark line, and that this is very likely to be correct is proved by 

 placing extremely thin sections of the pith of Clematis, or of bast 

 cells of Begonia in Canada balsam, which has the same refraction as 

 cellulose, and will, on filling the interspaces, cause the black lines 

 to disappear, while in anise oil, which is more highly refracting, the 

 phenomena are reversed, the dark striae being now the brightest. 

 To study the origin of the layers in unilateral secondary deposit, 

 the epidermis cells in young leaf buds of mistletoe, and the outer 

 wood cells in shoots and radicles of Conifers are adapted ; for con- 

 centric lamination we select bast and pith cells, or the stem of 

 Clematis Vitalha. 



Forms of Secondary Deposit. — The layers of concentric deposi- 

 tion are never complete, but portions of the primary cellulose case, 

 of greater or less extent, are free from the thickening. The forms 

 in which thickening occurs are many, but the principal may be 

 classed as annular, sinral, net-like, 2')orose and crihrose. 



1. Annular thickening of the cellulose membrane is seen oc- 

 casionally in parenchymatous and filiform cells, and in none more 

 beautifully than in the hyaline cells of a Sphagnum leaf; we have 

 also good examples in the parenchyma of Cactaceae, where the 

 thick rings extend inward like a plate. The rings vary in their 

 distance from each other, and also in their inclination to the axis 

 of the cell, and we may notice in Tradescantia, Balsamina, &c., a 

 gradual transition into the next form. 



2. Spiral thickening is often in form of homogeneous thickening 

 on one side of the cell ; it is well seen in the elaters of Hepatic^, 

 in Sphagnum leaves, and in the epidermis of anthers in Cucurbi- 

 taceee and Liliaceae, in the seed coat of Salvia, Taraxacum, Pole- 

 monium, &c., and in the vessels of Hyacinth, Arum, &c. Their 

 direction is usually to the left, but sometimes we find them turn to 

 the right and left in the same cell, as in the wall of the anthers of 

 pumpkin. The spiral band is constructed precisely as the thicken- 

 ing layer of the whole cell, having a primary layer next the lining 

 of the cell, an internal tertiary and middle secondary layer, and 

 these may be best seen in the large bands of the Balsam. In the 

 bast cells of Apocynacese and Urticeee, of the larch, and in the 

 outer part of the annual ring of Conifers, the spiral thickening 

 layers are deposited in bands, with numerous unthickened striae 

 between, and these were mistaken by Agardh and Meyen for a 



o 2 



