CHAPTER VII. 



THE VASCULAR OK CONDUCTING SYSTEM. 



/. GENERAL CONSIDERATIONS. 



In every highly-organised plant, substances of various kinds are con- 

 stantly being moved from place to place, at any rate during the periods 

 of active vegetation ; the nature and amount of the materials thus 

 transported varies greatly at different times. Before dealing in detail 

 with the paths along which this translocation or conduction of materials 

 principally takes place, we must first consider the nature and physio- 

 logical importance of the materials themselves. 



In land plants the most energetic transportation of materials takes 

 place in the so-called transpiration current ; this current serves mainly 

 to replace the moisture that escapes by transpiration, and also, to a smaller 

 extent, to supply water to the several tissues for nutritive purposes 

 and as "water of imbibition." The various inorganic and organic food- 

 materials travel, either in this transpiration current or independently, 

 from the absorbing organs to the photosynthetic tissues and to all the 

 organs that are engaged in active growth. In green plants which 

 alone need be considered for the present it is especially the mineral 

 salts or " ash-constituents " that are transported in this way. Lastly, 

 starch, sugar and other carbohydrates, fats, amides, proteins, in short, 

 plastic or synthetic materials generally, are constantly migrating from 

 photosynthetic tissues and from organs of temporary storage to growing 

 regions and permanent storage-tissues in fact, to every part of the 

 plant-body where such materials are required. 



The movements of these diverse substances are altogether regulated 

 by the momentary requirements of the plant and its component organs. 

 Thus materials travel from the root into the stems and leaves, from 

 storage-tissues into all parts of the shoot or root, from foliage-leaves into 

 buds, flowers, ripening seeds or fruits, growing roots and so forth. It 

 is obviously inadmissible to speak simply of an ascending current of 



