304 Maugham . — On the Mechanism of 
to be found in the case of the relations of the sieve-tubes of Cuscuta to those 
of the host plant. 
Details have been investigated more especially by Peirce 1 and by 
Thoday (Sykes ). 2 The latter found that after the haustorial cells of the 
parasite had penetrated into the vascular tissue of the host, a very close 
union was effected by some of them with the sieve-tubes. By absorption 
of the end walls the protoplasm of these haustorial cells eventually came 
into actual contact with sieve-fields or sieve-plates. 
As the haustorial cells so united themselves take on the structure of 
sieve-tubes, and are in connexion with the phloem of the parasite, there 
results a continuity of sieve-tube protoplasm between the two plants. 
Here it may be stated that experimental investigation by means of the 
osazone method for locating sugars in plant tissues 3 has shown that the 
sieve-tubes of the host, and the haustorial elements united with them, may 
both contain greater amounts of sugar than occur in the other tissues. 
These results, details of which will, it is hoped, be incorporated in 
a later paper, go some way towards confirming the correctness of the view 
here taken as to the role of sieve-tubes in sugar conduction, and help to 
emphasize the importance of protoplasmic continuity for effecting trans- 
location. 
Albuminous compounds present in the contents of the sieve-tubes may 
serve to increase the adsorptive capacity of these cells, and thus may add 
to their power of accumulating sugar. (Cf. Fig. i.) 
The advantage to the plant of close association in a common path of 
such important metabolites as carbohydrates and nitrogenous organic 
bodies scarcely calls for comment. 
The continuity of protoplasm which occurs in graft hybrids 4 might 
also be interpreted on lines similar to those taken in the case of Cuscuta and 
its host. 
Another point calling for some consideration is that the protoplasm 
itself can scarcely be exactly the same in constitution and properties 
throughout the plant . 5 
The very existence of cell differentiation indicates that the protoplasm 
of a cell of one type differs in some respect from that of another. 
The exact nature of these protoplasmic differences can only be 
surmised, but their results are often easy to observe. In many cases they 
3 Peirce (1893). Cf. PI. XIII, Fig. 7 , and PI. XIV, Figs. 14 , 17 , and 18 . 
2 Thoday (Sykes) (1911). It is fair to state that in this paper no claim is made to the effect 
that absolute continuity of protoplasm is established (cf. pp. 671 , 672 ). The association is, however, 
extremely close. 
3 Mangham (1915). 4 Hume (1913). 
5 Striking examples of differentiation are to be found in the case of the sporangiophore of 
Pilobolus i which actively excretes water, and in the case of nectaries, from which sugary liquids 
exude. 
