286 MANUAL OF BOTANY 



with two appendages or flagella at one end. These flagella, like 

 the rest of the cell, are composed of protoplasm, and are espe- 

 cially capable of vigorous movement, showing a certain diffe- 

 rentiation with that object. Other naked masses of protoplasm 

 are not furnished with these flagella, but possess much shorter, 

 more delicate threads known as cilia, which are placed either 

 all over their surface as in the case of the zoocoenocytes of 

 Vaucheria, or form a ring near one end, as in the zoospores of 

 (Edogoniuin (fig. 666, c). In many of such naked cells there 

 is a specially coloured corpuscle, usually red, known as an eye 

 spot. In others a special vacuole in the protoplasm shows a 

 rhythmical enlargement and contraction. This is known as a 

 contractile vacuole. It recalls a similar structure in the lowest 

 animal organisms. 



The Cell- wall. — We have seen that in most cases the first 

 evidence of the vital power of the protoplasm is the formation of 

 a membrane which surrounds it on all sides. This is known as 

 the cell-wall. It is chiefly composed of a substance known as 

 cellulose, with which in most cases other bodies known as 

 pectoses are more or less closely incorporated. Cellulose has a 

 very complex molecule, into which only the elements carbon, 

 hydrogen, and oxygen enter, the latter two existing in the same 

 proportion as they do in water ; this body is consequently a 

 member of a group of svibstances known as carbohydrates. Its 

 chemical formula is often written CcH.oOj, though probably it 

 should be some multiple of this. Sugar and starch are other 

 members of the same group. 



Cellulose is a colourless transparent substance, easily per- 

 meable by water and by substances in solution in it. It can be 

 stained violet by iodine in the presence of sulphuric acid or 

 chloride of zinc. 



/ The solid portion of the cell-wall contains a varying quantity 

 of water. Various views have ieen held as to the way in which 

 the latter is combined with the other constituents. According 

 to Nageli the particles of cellulose, known as micellae, are of 

 crystalline form, the long axes of the crystals being arranged at 

 right angles to the surface of the wall. Each micella is sur- 

 rounded by a thin film of water. According to Strasburger the 

 particles of cellulose are arranged in a network, water occupying 

 its meshes. In either case the quantity of water is capable of 

 considerable increase or diminution, and the wall can be made 

 to swell up by causing it to imbibe more fluid. This can be 

 brought about by exposing it to the action of strong acids, such 

 as sulphuric acid. 



