FUNCTION OF SIEVE-TUBES 



333 



protein compounds, which, moreover, is generally of a more or less 

 watery consistency. It is just where the solution is slimy as in the 

 Cucurbitaceae that the sieve-pores are unusually wide. Another 

 strong argument in favour of the protein-conducting function of the 

 sieve-tubes is derived from the disposition of these structures in the 

 plant-body. For, like the tracheal elements, the sieve-tubes are arranged 

 in continuous strands, which traverse all the organs that have to be 

 supplied with protein-compounds ; in particular, they link up the foliar 

 mesophyll, which is very probably the site of the most active protein- 

 synthesis, with the regions where protein com- 

 pounds are in greatest demand. It appears, 

 further, that the total cross-sectional area of the 

 leptome-strands in a particular organ which is 

 an approximate measure of the total number of 

 sieve-tubes in general corresponds very closely 

 to the relative quantity of protein-material that 

 passes through that organ. If, for example, the 

 transverse section of a tendril of the Vine be 

 compared with that of a leafy branch of the same 

 plant, the leptome is always found to be much 

 more massively developed in the last-mentioned 

 organ (Fig. 131). On the other hand, a similar 

 comparison between a vegetative stem or branch, 

 and an inflorescence-axis or peduncle of the same 

 plant, often demonstrates that the increased supply Small portion of a tan- 



gential longitudinal section 



of protein-compounds required for the purpose of through the secondary ie P - 



. . tome of the stem of Vitis 



pollen- or seed-production may necessitate a cor- vini/era, showing one of the 



. . nil i a oblique septa of a sieve-tube. 



responding enlargement of the leptome-strands. A a, b, the two adjacent seg- 



. . pi- merits of the sieve-tube. The 



single instance may be cited in support of this living contents, contracted 



, . owing to the action of alcohol, 



Statement. It the CrOSS-SeCtlOlial area Of the are seen in yl, adhering to the 

 . . . . . TT . septum and sending forth 



leptome in a yearling vegetative twig of the Hazel blunt processes through an 

 {Corylus Avellana) be denoted by the figure 100, a, a starch grain, x hod.' 

 the corresponding value for this tissue in a floral 



axis, bearing two staminate catkins, was in a given instance found to be 

 equal to 146. It is also significant, that sieve-tubes of large calibre are 

 present in large numbers in the walls of the insect-catching and -digesting 

 pitchers of Nepenthes. Increased demands upon the conducting capacity 

 of the leptome may, in certain circumstances, react not only upon the 

 quantitative but also upon the qualitative development of the sieve-tubes. 

 Among climbers, for example, the factors which have been referred to above, 

 in connection with the organisation of the water-conducting system, exert 

 an equally marked influence upon the development of the sieve-tubes ; 

 attention has indeed already been drawn to the unusual width of the 



Pro. 135. 



