INTRODUCTION 9 



Apart from these functions the cell, for growth 

 and reproduction {q. v.), divides up and becomes 

 differentiated. The work of absorption, osmosis, 

 synthesis of compounds, respiration, enzyme forma- 

 tion, excretion referred to, are only part of the 

 activities of the cell. It also exhibits contractility — 

 the power of perceiving and responding to stimuli, 

 e.g. in the arrangement of chlorophyll granules, 

 owing to too intense light, along the cell-wall. 



The cells together form a series of tissues which 

 have a definite form and arrangement, and are 

 marked off at different parts of the plant body, e. g. 

 a stem, to serve different purposes. 



At the growing point the tissue is a primordial 

 meristem, undifferentiated, with all the cells alike. 

 A section further down shows a ring of protoderm, 

 and, within the ground meristem, strands of pro- 

 cambium. Around the exterior, at a still more 

 advanced stage, is an epidermis, and the procambium 

 has become differentiated into an outer phloem, an 

 inner xylem making up the vascular bundle, with 

 cambium and secondary tissue between. The ground 

 meristem is elaborated into a primary cortex, peri- 

 cycle of bast fibres, primary medullary rays, and 

 medulla or pith in the centre. 



These are elaborated still further in the formation 

 of a thick woody stem, when the primary meristem 

 becomes primary or secondary permanent tissue, 

 with or without secondary growth. 



The epidermis serves to protect the plant against 



