ELEMENTARY OKGANS. 



121 



would crystallize at once ; but for the others, it is necessary that the elements which 

 have a reciprocal affinity should be united in proper proportions. In all cases, it is 

 only during life that this crystallization is carried on, for the crystals are found in 

 special cellular tissues, the forms of which determine theirs ; the same salt being, 

 in fact, found to crystallize very differently according to the tissues in which it is 

 formed. 



The crystals contained in the cells are either solitary or clustered ; in the latter 

 case they are grouped into radiating masses (fig. 672), or bundles of parallel needles 

 (fig. 673) named raphides (R); and they may often be seen escaping from the cells (c) 

 when the tissue containing them is dissected under the microscope. Finally, the 

 cells and even the intercellular spaces often contain silex, one of the most abundant 

 of minerals, which constitutes sand and flint; this silex even encrusts the tissues of 

 certain plants, and notably the straw of Graminece. Certain mineral concretions 

 are observable in the leaves of some Urticew ; if the leaf of a Nettle be viewed 

 with a lens, transparent spots may be distinguished ; this is due to the presence of 

 calcareous particles deposited in the outer cells, to which Weddell has given the 

 name of cystoliths. These cystoliths differ from the crystals represented in fig. 672, 

 in being deposited in calcareous layers around a nucleus formed at the expense of 

 the cell-wall, which has been pulled aside by the accumulation of mineral matter, 

 and has lengthened into a very delicate pedicel from which the cystolith is suspended. 

 This formation may be compared with that of stalactites. 



Epidermis. Before treating of the anatomy of the fundamental organs, we shall 

 describe the epidermis, which covers the surface of the vegetable. If the leaf of a 

 Lily or Iris be torn, a shred of transpa- 

 rent colourless membrane is detached from 

 one of the fragments, together with some 

 cellular tissue, filled with green chlorophyll; 

 a simple lens shows on this membrane 

 several parallel (fig. 676) or reticulate lines 

 (fig. 677) and small, more opaque spots. 

 Under a microscope, it is seen to be com- 

 posed of large flat cells, which may be 

 hexagonal or quadrilateral, or irregularly 

 waved, and which contain a colourless 

 liquid ; their lateral walls are closely 

 united, whence the solidity of the epider- 

 mis ; their lower surface slightly adheres 

 to the subjacent cellular tissue; their 

 exposed walls are usually thicker than the others, and may be flat or raised in the 

 centre, according as the surface of the epidermis is smooth or rough. 



In most cases, the epidermis is composed of a single layer of cells ; when there 

 is a second, it is usually formed of much smaller cells. The lateral walls of all the 

 epidermal cells are not contiguous ; many of them present interspaces, occupied by 

 little bodies resembling a button-hole with a double rim or border (figs. 676, 677), 



st 



676. Lily. 

 Epidermis and stomata. 



677. Balsam. 

 Epidermis and stomata. 



