18 OEGANOGRAPHT. 



The dimensions of prosenchymatous cells generally afford a 

 striking contrast to those of the parenchymatous, for while we find 

 that their transverse diameter is commonly much less, averaging 

 about pr^Q- of an inch, and frequently not more than ^^^^, they be- 

 come much more extended longitudinally, some having been 

 measured as much as I of an inch long, and according to Schleiden, 

 those of the inner bark are often four, five, or more inches in 

 length. The prosenchymatous cells of the wood and inner bark 

 of trees generally vary, however, from about the ~ to the ^ of 

 an inch in length. 



Those cells again which have an unrestrained development are 

 frequently also far more extended in length. Thus, the cells of 

 which cotton is formed (Jig. 132, a) are sometimes as much as one 

 or two inches long, while in some of the Cryptogamous water 

 plants, as Chara, cells occur several inches in length. 



3. The Cell-membrane or Cell- wall. — a. Its Chemical Pro- 

 perties, — The membrane of which the cell is composed consists of 

 the substance called c ellulose or cellulin, and as all plants and all 

 parts of a plant are formed essentially of cells variously modified 

 and combined, this substance must be considered as the funda- 

 mental material of the plant. When pure it is a ternary com- 

 pound of c arbon, hy drog en, and oxygen ; of which the latter two 

 exist in the _same _pr oj>ortions as m water . Hence it may be 

 considered as consisting of carbon "and the elements of water. 

 Some observers describe the cell-wall as formed of two mem- 

 branes, the outer having the composition as just described, and 

 t he in ner, which is termed the Prim ordia l utricle^ cont aining 

 nitroge n, in addition to_carbon, oxygen, and hy drogen. In ac- 

 cordanc^ with the view's generally entertained, we have described 

 the so-cal led pr imordial utricle as one of the contents of cells. 

 (Seel):26Tr'^^ "~ ' ; 



Ce llulose is insoluble in both cold a nd boiling water ; also in 

 alcohol, ether^ and dilute aci ds^ and almosj b insoluble in w.£ ak al- 

 Icalilie.Si2iutions. B y the action of cold concentrated sulphiiric 

 acid upoii^cellulos e_ajtreacly-lookrng liqui d i s produced , which is 

 converted, after d ilution and boiling, first into dextrin , and t hen 

 into ^ape si^ar. Wh en iodine a^ndr~sulph uric a^id_ are_aj)plie^ 

 to celIurose7 it assum es an ihH lgo blu e coTourv w hich is re ndered 

 more eyideiit if the~ s ulph uric a cid be previoiTslj d ilutied with 

 water (the best proportions being one part of the latter to three" 

 of the former). A sim ilar blue colour is also produced when 

 cellulose is moistened~wit h aTsolution o f chlo ricle of zinc, io dine, 

 and^lodide of poTas^^L JVLohl has also shown that cellulose 

 wiirassume a blue colour if it be thoroughly imbued with iodine, 

 and afterwards moistened with water. The blue colour will 

 not be produced however, under such circumstances, when we 

 operate upon the cell-membrane of very young cells ; hence it 

 is probable that thejoung cell-membrane may be composed of a 



