44 THE PLANT CELL. 



general sliape of the synclinal and anticlinal surfaces, when cut 

 by a plane passing through the longitudinal axis of the bud, 

 would thus be parabolic, the two sets of parabolse cutting one 

 another at right angles, and the foci of both sets of curves would 

 be within the area of the growing point of the shoot. 



The contents of the young cells consist, as has been mentioned, 

 of protoplasm, nucleus, sometimes plastids, and cell-sap, in which 

 latter certain salts are held in solution. The plastids (when 

 jtresent) manufacture staieh, not quite in the same manner as the 

 chloroplasts, Init from certain elaborated materials (sugar) brought 

 to the cells from the leaves, and from the starch thus built up the 

 protoplasm is able to manufacture cellulose, for the purpose of wall 

 formation. The production of cellulose is, however, not a simple 

 matter, since it has been shown that in the production of the cell- 

 plate, or partition wall dividing a cell into two during the later 

 phases of cell-division (see Chap, viii.), the protoplasm undergoes an 

 almost direct transformation into cellulose by the .splitting off of its 

 carbohydrate molecule, the remaining proteid and amine portions 

 being then free to combine with carbohydrate derived from other 

 sources in the cell. Moreover, it has been found that manj^ stages 

 ordinarily exist between protoijlasm and cellulose, and that starch 

 before it can be utilised must first be converted into dextrins and 

 sugar by the agency of enzymes, and it is probably this sugar 

 which is made use of by the protoplasm. In the latter process 

 oxidation possibly has a large share. The unlignified cell-wall 

 has a large amount of pectose in its composition, pectose havinj 

 the same generic formula as cellulose; the middle-lamella, in fact, 

 consists of calcium pectate. 



Physically speaking, growth of the cell-wall takes place in two 

 ways, viz. : — 



(a) Growth by intUSSUSCeption, fresh particles of cellulose being 

 intercalated between those already existing. 



{!)) Growth by acePetion— '.«., fresh layers of cellulose are laid down 

 one after the other, somewhat after the manner in which crystals increase 

 in size. 



Both these processes are going on together in the cell ; growth 

 of the wall in surface-area being effected by intussusception, 

 whilst growth in thickness of the cell-wall proceeds by accretion. 



ifTthis connection it is necessary to give a few instances jf 

 the secondary thickening of the cell-wall by accretion. Tk 



