12 AUSTRALIAN BOTANY. 



active ones continually form at either extremity. Upon 

 the uninterrupted continuance of this process depends their 

 vitality. When it ceases, the individual dies. This occurs 

 either at definite periods, as in annuals (plants living one 

 year, or a part thereof) and biennials (dying in the second 

 year) ; or not, as in perennials, plants which vegetate 

 several years, viz. from three or four to a thousand and 

 upwards. 



Every part of plants being formed of cells, either as such 

 or with a metamorphosed (changed) state, they are said to 

 consist of cellular tissue. Wherever this is exposed to the 

 air, whether root, stem, leaves, or any other part, it is 

 covered by a skin or epidermis, and closely packed cells 

 inwardly, so as to prevent contact of the interior with the 

 outer air except through very minute openings of peculiar 

 structure, called stomata or pores. These latter are 

 principally found on the soft and green parts, as the 

 under side of the leaves, the flowers and fruits, and also 

 the stems of herbs, grasses, etc. 



According to the locality in which cells are situated, 

 their functions differ in such a way as to ensure the 

 required results. Thus in the leaves they are fitted to 

 absorb carbonic acid with the air and moisture of the 

 atmosphere. The former they decompose under the 

 influence of light. Its carbon is retained and changed 

 into wood substance. The charcoal of the wood is pro- 

 duced almost entirely from this source. The oxygen, so 

 indispensable for animal life, is again exhaled. This 

 accounts for the invigorating effect of a walk in the woods 

 or green fields. 



At the tips of root fibres, again, the cells are fitted to 



