Il6 CELLULAR TISSUE. 



As was stated in Sect, i, ihe'w /orm is extremely various, and we may here 

 distinguish as the chief types the iso-diametric, or s/wri forms, and the elongated — 

 fibrous cells, fibrous-parenchyma (' Prosenchyma '). The further distinction of forms, 

 to which during a certain period much energy was devoted^, has at the present 

 day hardly even an historical interest. However certain definite forms, which are 

 characteristic for definite single cases, must be mentioned. 



As is the case in cellular tissue generally, the special structure may be taken 

 into consideration, and the distinction may be drawn between thin-walled and thkk- 

 wallcd parenchyina, according to the relative development of the membrane on the 

 one hand, and of the protoplasm and cell-contents on the other ; but this holds only 

 in extreme cases. In the distinction of subdivisions, the manner of connection of 

 the cells one with another is taken into consideration as one of the characteristic 

 special relations of structure. 



Thm-walled parenchymalous cells are in most plants the organs of the process of 

 assimilation, and the storehouses of its first products ; besides having a relatively 

 thin membrane they are therefore usually distinguished by their contents ; viz. 

 assimilating chlorophyll, and the most widely-spread direct product of assimilation, 

 starch. According to the preponderance of one or the other of these parts, we may 

 speak shortly of chlorophyll parenchyma, starch parenchyma, in many other cases of oil- 

 co7itaimng parenchy?na, &c. The parts of plants which contain chlorophyll, and in 

 which reserve products are laid up, e. g. especially the leaf, cortex of stems, and 

 Rhizomes, are the places where these cells occur in large masses. 



In contrast to those characterised by the parts of the protoplasm and contents 

 appearing as above described, there are other thin-walled parenchymatous cells, in 

 ■which, within a protoplasmic sac, which is usually very delicate and slightly de- 

 veloped, all the solid constituents of definite form diminish till they disappear entirely 

 before the cell sap ; this sap fills almost the whole of the cell, and is watery, 

 or contains very thin mucilage. This may accordingly be termed sap-parenchyma. 

 This is wide spread, and as ' aqueous tissue ' has recently been thoroughly described 

 by Pfitzer ^ in many thick long-lived foliage-leaves, in which it is situated beneath 

 the epidermis (hypoderma), forming as it were layers strengthening the latter, as 

 in the Pleurothallidese, Bromeliacese, Ilex, Nerium, &c. ; or it appears as a middle 

 layer of the leaf, and is surrounded by chlorophyll-parenchyma, as in many succulent 

 plants, e. g. species of Aloe and Mesembryanthemum, and in the leathery leaves of 

 species of Callistemon, Hakea, &c., which will be more fully described in Chap. IX. 

 It occurs in specially large masses in parts without chlorophyll which are rich in 

 inulin or sugar, such as tubers and roots of Compositse, Campanulace^, Beta, &c. 

 The cells in question are characterised by their contents, which are almost perfectly 

 transparent and fluid, being sometimes watery, sometimes (species of Aloe) muci- 

 laginous. Their chemical constituents are exactly known only in single cases, as 

 in the above Compositse and Beta, they cannot therefore at present be used in 

 distinguishing them generally. 



* Hayne. in Flora, 1827, II. p. 601. — Meyen, Phytotomie, p. 63. — C. Morren, Bull. Acad. 

 Bruxelles, torn. V. No. 3.— Compare Mohl, Veget. Zelle, p. 15. 

 '' Prinysheiin s Jahrb. ^'III. p. 16. 



