THE ANGIOSPERMAE : STEMS 



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derivative cells, which are not yet permanent in character and retain the 

 meristematic quality of repeated division. 



The apical meristem is thus not uniform in its nature. The initial 

 meristem alone is completely undetermined, while the determination of the 

 destiny of its products begins while they are still part of the meristematic 

 tissue as a whole. 



A cell which is destined for differentiation into permanent tissue retains 

 the meristematic character for some time and may at first be indistinguishable 

 from one of the initial cells, repeated division taking place, as shown by the 

 frequency of mitotic figures. 





* » Leaf rudiment I 



Meristem initials 



Leaf rudiment II 



Fig. 836. — Zebrina peudula. Monocotyledon. Trans- 

 verse section of the apical bud showing in the centre 

 the growing point of the stem surrounded by two 

 leaf rudiments. Note the rapid lateral extension of 

 the leaf rudiments characteristic of Monocotyledons. 



Physiological changes are taking place, however. Protein synthesis, 

 which is characteristic of the meristematic state, is giving place to carbo- 

 hydrate synthesis, and the accumulation of osmotically active sugars is 

 probably responsible for the growth of cell vacuoles, the first visible evidence 

 of change. The cell wall also changes, becoming thicker and passing through 

 a highly hydrated stage, known as the amyloid stage, in which it is extensible 

 but not elastic. This stage is ver}- transient, but while it lasts the internal 

 turgor pressure in the vacuoles causes the cell to enlarge considerably, 

 with further enlargement of the vacuoles, which may now unite to occupy 

 the whole central part of the cell. This is the phase of maximum growth. 

 The cells become rounded off and small intercellular spaces appear between 

 them, which are at first full of watery sap, but later contain air. These spaces 

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