"CmaiJariS"^'] MtcToscopiG Stvucture of Plants. 35 



really has bad its origin at the corners of the cell, and extended to 

 each side. Porous and pitted cells are also familiar examples of 

 partial thickening. In some cases the cell-wall becomes absorbed, 

 and a channel of communication between the cells is formed. If 

 we have thickening occurring at certain parts of the cell-wall, 

 resembhng a porous or pitted cell, and if the primary cell-wall, 

 which is not covered by thickening matter, becomes absorbed, then 

 we have the peculiar cribriform cells formed, such as are to be seen 

 in the stem of the white gourd (Cucurhita Pepo). These cribri- 

 form cells form a connecting link with the punctated tissue of the 

 conifers and ordinary porous or pitted cells. In the Coniferae we 

 have the thickening deposit leaving rounded patches of the primary 

 cell- wall uncovered. As the thickening goes on the thickening 

 matter extends in and in, until a cavity, shaped like a plano-convex 

 lens, is formed, and leaving a small round centre-spot free fi'om 

 thickening matter. The same thing takes place on the other side 

 of the ceU-wall; so that, when the small portion of the primary 

 cell-wall becomes absorbed, we have a cavity, shaped hke a double- 

 convex lens, formed, and the cavities of the two cells freely com- 

 municate with each other. We see examples of a very similar 

 process in the tuber of the Dahlia, and still better in the scalari- 

 form tissue of Ferns. In ferns we have a tissue formed in the 

 same way as the punctated tissue of the pines, with this difference, 

 that in the pines the dots are circular, while in the ferns the 

 portion of primary cell- wall left uncovered is elongated, the thick- 

 ening occurring in bars, resembling the steps of a ladder : hence the 

 name scalariform given to the tissue. 



In some cases the thickening deposit in the interior of cells may 

 be made up of layers of different appearance. This is to be seen, 

 for example, in certain cells in the stem of the Common Bracken, 

 where we have the primary cell-wall and two differently coloured 

 layers of thickening matter inside. Besides differences in appear- 

 ance, we may have chemically different substances deposited in the 

 thickening layers of the cell- wall. Take, for example, the Charas 

 and Diatoms. In these we have mineral matter, as carbonate of 

 lime and silica, deposited in the cell-wall. There are many other 

 examples of silicious matter deposited in ceUs besides the diatoms. 

 The silicious matter deposited in the cells of the cuticle of the 

 Equisetums, and in the ceUs of grasses, .&c., is familiar to most 

 microscopists. In general the primary cell- wall remains very thin, 

 but in certain instances — as, for example, in the cells of certain sea- 

 weeds — the primary cell- wall becomes greatly thickened, a sort of 

 gelatinous degeneration occurring in it. We see the same thing 

 in a few cells of the endosperm of the Locust-Bean. In these 

 cases the primary cell- wall has been described as intercellular sub- 

 stance. This intercellular substance has given rise to a great deal 



