J20 CELLULAR TISSUE. 



thickened parts of the walls have no pits, and are delicately stratified with very 

 fine limiting layers ('Intercellular-substance'): they swell largely with water, but 

 without becoming gelatinous ; when water is removed they contract greatly in all 

 directions (measurements are wanting). In the soaked condition they show, in 

 transmitted light, a characteristic bluish white lustre. With Schultze's solution they 

 turn light blue^; after slight warming with potash they immediately turn deep 

 blue with solution of iodine in potassium iodide (this is the case in Sambucus, 

 Rumex, Lamium album, Cactace^, Nymphcea). 



In the same places which are occupied in many plants by cells thus remarkably 

 characterised by the above described properties, there are found in many others layers 

 of cells which differ more or less from these in their form and in the structure of their 

 elements. For instance, the cells of the Collenchyma of the stems of Cacti already 

 mentioned - differ from those described as typical, in their slight elongation, and in 

 the fact that the walls are thickened strongly and uniformly all round, and coarsely 

 pitted. Other single forms approach nearer to the thin-walled or sclerotic forms of 

 parenchyma, without its being possible to carry through any sharp distinction. It is 

 then to a great extent a matter of taste how far one will extend the term Collenchymd. 

 It is now used generally for the form of tissue here described as typical, though it 

 was originally proposed by Link ■' for the pollen mother-cells with their gelatinous 

 membranes, and was then transferred by Schleiden, at first half in joke, to the above- 

 mentioned collenchymatous cells of the Cactacese. 



From the collenchyma must be distinguished the thick-walled forms of paren- 

 chyma, the membranes of which are more or less lignified, and have thus become 

 hard and sclerotic. As the most typical representatives of tissue of this sort may be 

 brought forward the thick-walled cells of the secondary wood of Dicotyledonous 

 trees, which lay up starch periodically, and often prove themselves capable of divi- 

 sion in the case of wounds, or rather healing scars. This will be entered into in 

 more detail in Chapter XIV. In other places than that just mentioned sclerotic 

 cells are to be found widely spread : together with collenchyma and sclerenchynia 

 they form the strengthening apparatus of those parts, and they are connected with 

 both of these tissues by the most various transitional forms. No general specific 

 peculiarities of this tissue can be mentioned in addition to what has been already said ; 

 remarkable examples will therefore for the most part be mentioned in the chapters 

 w'hich deal with the distribution of tissues. Here we may briefly notice only one series 

 as being specially instructive, and as presenting difficulties in a sharp classification 

 of the tissues, viz., that of the sclerotic cells in the Ferns. In the large majority of 

 these plants there occur in stem, roots, and leaves thick-walled elements, sometimes 

 isolated, but usually in close and often in uninterrupted connection with one another, 

 and combined to form uniseriate or multiseriate layers or bundles ; these either lie 

 near the epidermis, or accompany or ensheath the vascular bundles. In the petiole of 

 the Marattiaceae they have the properties of collenchyma, as was above stated ; also 

 many bundle-sheaths, to be cited later, are directly connected with this tissue as regards 

 their structure. But in the large majority of cases (compare Fig. 48) the walls, which 



* Schacht, Lehrb. p. 195. 



* Compare Unger, Grundziige, p. 25; Schleiden, Anatomic d. Cactcen, p. 14. 

 2 Grundlehren d. Krauterkunde, II. p. 199. 



