576 TRANSLOCATIOX 



locating substance can be detected in them at any particular moment. For 

 all these reasons it is not surprising to find that, in contradiction to what 

 was formerly supposed, the occurrence of starch in the bundle-sheath or 

 pericycle does not indicate that these layers possess any special importance 

 or significance in translocation l . 



All substances are not translocated and accumulated with the same 

 readiness, and hence a partial separation of the different plastic substances 

 is possible ; thus in seedlings, starch, sugar and oil are mainly present in 

 the cortical and medullary parenchyma, while the proteids occur for the 

 most part in the phloem and especially in the sieve-tubes -, which are 

 especially well adapted by means of their open intercommunications for 

 the transference of these slowly diosmosing bodies. In general the amount 

 of proteid translocated is small, and hence the channels mentioned suffice for 

 all requirements, for whenever reserve-proteids are mobilized they usually 

 undergo marked decomposition and a great part of the nitrogen is trans- 

 located in the form of asparagin and other amides, which along with sugar 

 and starch are found in the fundamental parenchyma, although proteids 

 are probably subject to direct translocation in seedlings ?> . This separation 

 between translocating substances which meet again at their final destination 

 is never complete, and is by no means a general necessity, for in sieve-tubes, 

 endosperm tissue, fungal hyphae, &c., proteids and carbohydrates travel 

 together. 



The component elements of the phloem are, like those of the xylem, of 

 unequal functional value. The sieve-tubes seem to be the most active 

 conductory channels for all substances, whereas the cambiform cells are of 

 less value in translocation, and a subordinate importance only is attached 

 to the companion cells, and still less to the phloem parenchyma. The 

 elongated tissue-elements both of the wood and of the phloem form the 

 most important conducting channels, and special importance attaches to 

 the vessels or long tubes formed by the perforation of the transverse 

 walls of contiguous cells. The water-conducting tracheae are however 

 usually dead, whereas for the most part living cells and cell-fusions serve 

 for the conveyance of plastic materials. Hence the sieve-tubes lose their 

 conducting power as soon as they die, which occurs after a limited period 

 of activity in plants which exhibit secondary growth and form new sieve- 

 tubes from year to year. 



The mobilized products may undergo transitory deposition in the 

 process of translocation, and thus the occurrence of starch in the conducting 

 channels by no means indicates that carbohydrates are transferred in 

 this form. Other substances, however, including colloidal proteids, may 



1 Heine, Versuchsst, 1888, Bd. XXXV, p. 161 ; Schimper, 1. c., p. 757 ; Strasburger, 1. c., p. 487. 



2 Sachs, Flora, 1862, p. 297 ; 1863, p. 38. 



" Ffeffer, Jahrb. f. viss. Bot, 1872, Bd. vill, p. 538, and Sect. So. 



