TRANSLOCATION OF ORGANIC FOOD-SUBSTANCES 573 



and are at a later stage mobilized and transferred to the consuming organs, 

 while the removal of the translocatory products by artificial means may 

 induce premature depletion. Translocation is regulated to a very marked 

 extent by consumption, and hence a substance present in sufficient quantity 

 may undergo very rapid translocation until the plant's needs are satisfied. 

 Different substances may be transferred in opposite directions at the same 

 time and along the same channels ; indeed this occurs in every plant, 

 for the excretion of carbon dioxide and other substances proceeds simul- 

 taneously with the absorption of nutriment and of oxygen. 



When a seed germinates, the reserve food is conveyed from it to the 

 elongating root and shoot. Water and oxygen, as well as such inorganic 

 ash constituents as may be present in the soil, are absorbed and transferred 

 to the different parts of the plant from the commencement of germination 

 onwards. As soon as green leaves are formed these begin the photo- 

 synthetic production of organic food, and before long the whole of the 

 organic nutriment may be provided in this manner. In an annual plant 

 growth and the formation of new shoots, leaves, and roots, decrease 

 towards the end of the vegetative period, while large quantities of food- 

 material are consumed in the production of the fruit and are stored up in 

 the seed. The translocation of food-materials for this purpose becomes 

 more and more marked, while the activity of photosynthetic assimilation 

 gradually decreases, and ceases before the chlorophyllous organs are com- 

 pletely dead. In addition to organic substances large quantities of ash 

 constituents are stored up in the fruits, and the percentage of ash undergoes 

 but little increase in the rest of the plant subsequently to the period of 

 most active absorption and growth. 



In perennial plants a large portion of the assimilated material is con- 

 veyed to the permanent organs, such as the subterranean roots or rhizomes 

 of perennials, and to the stems of trees, while in spring a reverse current 

 conveys food to the developing buds. 



Water and dissolved food-materials necessarily follow the same 

 channel in a fungal hypha, whereas the tissue-differentiation in higher 

 plants is accompanied by a more or less marked division of labour, the 

 xylem conveying water and salts, the phloem plastic substances. The 

 phloem contains conducting tissue-elements which are as efficient and 

 important as those of the xylem, and when rapid translocation to far 

 distant parts is necessary, the food-materials pass almost solely through 

 the phloem, for transference takes place but slowly through the cortex and 

 pith. Hence as a general rule substances are translocated as far as pos- 

 sible by means of the phloem, and pass through relatively few parenchyma 

 cells to reach their destination, for it is only when the distance to 

 be traversed is small that plastic substances can be transferred through 

 parenchyma with sufficient rapidity to satisfy the needs of an organ 



