270 SCIENCE PROGRESS 



more natural that the sugars should take the same paths during 

 translocation as the less abundant proteid substances. 



Another piece of anatomical evidence in support of this 

 view is to be found in the presence of internal phloem in the 

 " Lindsaya" type of Fern stele. Inside the mass of xylem a few 

 sieve-tubes and parenchymatous elements occur, continuous 

 with the phloem of the leaf trace. As pointed out to me by 

 Mr. Tansley, it is difficult to understand the value of sieve-tubes 

 in this position if they only conduct albuminous materials. If 

 on the other hand sugars also travel in them, the centrally placed 

 sieve-tubes are admirable contrivances for supplying the carbo- 

 hydrate material needed to construct the large amount of thick- 

 walled xylem elements present. In the case oi Matonia sarmentosa 

 these sieve-tubes apparently become lignified. 



Movements of the Protoplasm. — The phenomenon of proto- 

 plasmic streaming calls for some consideration here. It has 

 been held by some investigators, notably De Vries, that this 

 plays an important part in mixing up the cell contents and 

 aiding their transference. But this movement, thought at first 

 to be of very general occurrence under normal conditions, has 

 been shown by Pfeffer and by Hauptfleisch to be, in most cases, 

 the result of the stimulus given on wounding the tissues and 

 not to occur so widely as was previously imagined. That such 

 movement of the protoplasm actually does increase the rate of 

 diffusion has been shown by comparatively recent experiments. 



Lecomte claimed to have seen protoplasmic streaming in sieve- 

 tubes themselves not only in the young elements but also in some 

 cases during functional activity. Strasburger only observed it in 

 the young stages and Czapek has stated that his own investiga- 

 tions show that the streaming ceased in the sieve-tubes of all the 

 plants examined as soon as the nucleus had disappeared and 

 the glistening slimy contents had formed. Thus, whilst it is 

 probable that the transport of assimilates in the parenchymatous 

 tissues is aided by movements of the protoplasm, it is doubtful 

 whether this phenomenon is of any importance when considering 

 translocation in the sieve-tubes. 



The Closing of Sieve-tubes. — It has been observed that in 

 many plants, of which the vine is one example, a closure of each 

 sieve-tube element is effected at the end of the annual growing 

 period by the deposition of more callus upon the sieve-plate. 

 In this way the pores become constricted and lengthened and 



