ELEMENTARY STRUCTURE. 35 



posed of oxalate of lime, others state them to be phosphate of 

 ^ime. Carbonate of lime crj'stals also frequently occur in plants, 

 as in many of the Cactacese. Crystals of tartrate of lime are 

 found in the old stems of some Cactacese ; those of sulphate of 

 lime in the Musacese ; and those of other composition occasionally 

 occur. 



Raphides, as might be supposed from their varying composi- 

 tion, assume different crystalline forms ; thus some are acicular 

 or needle-shaped (as we have already seen), with pyramidal 

 ends ; others, as those of oxalate of lime, crystallize in octahedra, 

 and in right-angled four-sided prisms ; carbonate of lime crystals 

 assume a number of forms, but most commonly that of the 

 rhombohedron ; and other crystals assume a variety of other 

 forms according to their composition. 



Section 2. Of the Kinds of Cells a:n*d their Connexion 



WITH EACH other. 



I We have already seen that when cells are of such forms that 

 combined together they mei'ely come in contact without percep- 



' tibly overlapping, they are called parenchyinatous ; but that when 

 elongated and pointed at their ends, so that in combination they 

 overlap each other, they are termed jprosenchymatous. We have 

 also seen that such extreme forms are connected by all sorts of 

 transitional ones. Formerly, all elongated organs found in 

 plants were supposed to have an entirely distinct origin fi'om the 

 cells, and were described under the names of Woody Fibres, and 

 Vessels or Ducts, but it is now known that these are all derived 

 originally from ordinary cells, and owe tiieir peculiar appear- 

 ances, either to various modifications in shape, which the latter 

 undergo in the course of growth, or to their combination and 

 union with each other. This common origin of the Woody Fibres 

 of old authors and the Vessels or Ducts, with the cells, is proved 

 by the fact, that gradual transitional forms from the one to the 

 other may be commonly observed ; and also by tracing their de- 

 velopment, when it will be found that all these organs, however 

 modified in shape and appearance, are derived originally from 

 the ordinary cell. All the observations made previously, there- 

 fore, as to the chemical and general properties of cell-membrane, 

 as well as to the mode of growth and deposition of secondary 

 layers, apply equally to the Vessels or Ducts. We have already 

 stated this to be the case with regard to the Woody Fibres, 

 which we have spoken of under the name of Wood-cells. By 

 the combination of the different kinds of cells, we have various 

 compound structures formed which are called Tissues : these we 

 now proceed to describe. The most important and the most 

 abundant of them all is 



1. Parenchyma. — This is composed of comparatively thin- 

 d2 



