34 Microscopic Structure of Plants. [*'j°o"S,^anTiwo?' 



thickening is to be met with in pollen grains, some of them present- 

 ing very remarkable appearances. In others the external thickening 

 may be undeveloped, as for example in the pollen grains of Zostera, 

 in which the outer covering, the Extine is wanting. By far the 

 most famihar examples of thickening are to be found in the interior 

 of cell-walls. Take, for example, wood cells. Here we have a very 

 good example of thickening, or in the hard cells of certain fruits. 

 This thickening in the interior of cells may be either regular or 

 irregular, but in by far the most numerous cases the thickening is 

 irregular. Certain parts of the primary cell- wall become more or 

 less thickened, while at other parts the cell- walls remain unchanged. 

 The familiar examples of spiral deposits and annular rings of 

 thickened matter are well known to most observers. In some parts 

 we have more or less marked spiral fibres apparently coiled up in 

 the cell. This is a thickening deposit, but it is so loosely attached 

 to the primary cell-wall that in many cases it can be very easily 

 detached from the cell-wall. The same may be seen in cells con- 

 taining annular rings. These may be in some cases quite easily 

 detached, and may even be found loose in the cell by which they 

 are formed. Often we have these different forms of thickening 

 combined, thus forming cells the walls of which may be partly 

 annular, partly spiral, or reticulated in various ways. These 

 varieties of cells can be very easily obtained, and many plants, such 

 as balsam, asparagus, rhubarb, Indian corn, &c., may be used to 

 demonstrate them. The cells containing thickening deposits can 

 often be isolated by boiling in nitric acid. Boiling in nitric acid 

 has, however, the disadvantage of rendering the parts yellow, and 

 often exercising a very considerable solvent actio u where it is not 

 wanted. Many beautiful preparations can be got by boiling thin 

 slices or even thin parts of plants in caustic potash. This renders 

 the tissue more transparent, without destroying the clearness of the 

 thickening deposit. In this way beautiful demonstrations of the 

 elongated spiral cells of certain plants can be made, the spiral cells 

 standing clearly out among the transparent tissues in which they 

 He. 



Many of the examples of irregular thickening occurring in the 

 inside of cell- walls are very curious and interesting. In the epi- 

 dermal and sub-epidermal tissues of plants we have some very 

 remarkable examples of thickening. In many of these cells the 

 thickening only occurs at those parts where neighbouring cells 

 meet. These angles or corners seem to be strengthened by the 

 addition of a sort of buttress of thickening matter. In the petiole of 

 Begonia this thickening can be well seen in patches at the corners. 

 In the petiole of the Ivy the same thing is observable, but the 

 thickening has gone on to such an extent that it requires some 

 little care to show that the deposit is not uniform, and that it 



