K.— BOTANY 187 



as the cambium. This cambium consists of cells which grow and 

 divide, as did the cells of the shoot apex which were thus responsible 

 for the original growth of the shoot axis, and the cambium cells prove 

 to be very closely related to the growing (meristematic) cells of the 

 apex. 



At the shoot apex two processes can be seen to follow one another in 

 close succession. In this region the crowded leaf primordia, with the 

 youngest nearest to the apex, are evidence that surface growth is in excess. 

 Growth is here proceeding in a beautifully ordered manner, throughout 

 a mass of similar meristematic cells. The shape of each cell is being 

 determined by the mutual pressure their expanding semi-fluid contents 

 exert upon each other, their plastic walls yielding readily to pressure, 

 whilst cell expansion is due to the increase in amount of living protoplasm. 

 The cells remain small because after a certain increase in size each cell 

 divides into two new cells which behave in the same way. Just below 

 the apex the leaf primordia are being dispersed along the shoot axis, as 

 they increase rapidly in size, because here cell expansion by vacuolation 

 is taking place, mainly in a longitudinal direction, in association with a 

 series of cell divisions in which the new cell walls are formed at right 

 angles to the length of the shoot axis. At this stage of cell growth, cell 

 extension is rapid and largely due to the intake of water. The cell wall 

 resists this rapid extension, so that neighbouring cells are no longer in 

 mutual contact over their whole surface. Their walls separate from one 

 another at all angles of contact, so that an intercellular space system now 

 develops. This is at first full of sap but rapidly fills with air — a change 

 that must exert a profound influence upon the further progress of growth 

 in this tissue. In all probability the great longitudinal extension of the 

 shoot tissues, which are composed of such vacuolating, dividing cells, is 

 attributable to the properties of the cellulose wall, which resists expansion 

 in some directions more than in others. 



Before shoot extension begins, the leaf primordia are crowded at the 

 apex, with no indications of internodes between them. When cell 

 extension and vacuolation, with continued cell division, lead to the 

 development of the internode, all the cells of the leaf primordium and 

 axis do not vacuolate simultaneously. In the median plane of the flattened 

 primordium and in continuity with this, in the axis, in a region between 

 vacuolating cortex and pith, the cells remain meristematic, continue to 

 increase in size by increase in protoplasm and remain in contact with each 

 other over their whole surface so that intercellular spaces are absent. 

 These meristem cells are therefore behaving just like the apical meri- 

 stematic cells, but they are now embedded in vacuolating dividing tissues 

 which are extending mainly in a longitudinal direction. As a result 

 their plastic walls are drawn out in a longitudinal direction, and we 

 distinguish as procambium this strand of meristematic tissue in 

 the leaf primordium together with its prolongation into a hollow 

 cylinder or network in the extending internode. This procambium is 

 however nothing but meristem such as is present in the surface of the 

 shoot apex, which is still left as meristem when neighbouring cells 

 vacuolate. 



