Guilliermond - Atkinson 



176 — 



Cytoplasm 



the longitudinal axis, while the small vacuoles are accumulated 

 against the proximal wall of the cell (Mangenot). 



This polarity of arrangement, which becomes more marked as 

 digestion becomes more active, clearly indicates that these cells 

 are traversed by a continuous flow of material caused by protein 

 digestion (proteolesis) at the level of the extremities of the 



tentacles. Thus, the "aggregated" state 

 of the vacuoles, i.e., their irregular dis- 

 position, seems to correspond to differ- 

 ing physiological conditions — to a secre- 

 tion in the cells which cover the ex- 

 tremity of the tentacle, to an absorp- 

 tion in the cells of the stalk — but both 

 these processes indicate the passage of 

 a current across the cells. This same 

 arrangement of vacuoles is found in 

 young sieve tubes in the angiosperms in 

 which the fragmented and polarized 

 vacuoles are very polymorphic. In these 

 cells, large vacuoles and small filamen- 

 tous or reticulate vacuoles are found. 

 They are also found in the conducting 

 elements of the Laminariales and the 

 Rhodophyceae (Mangenot) (Fig. 123). 

 Recent work seems to suggest that 

 the vacuoles undergo a similar frag- 

 mentation each time that the cells are 

 in the process of active secretion (Man- 

 genot, Mile. Py, Thomas, Guillier- 

 mond). The reason for this is not yet 

 known. 



Another phenomenon to consider 

 here is that of the frequent changes in 

 form of vacuoles in many cells. The 

 observation of a species of Saprolegnia 

 in van Tieghem and Le Monnier cells, 

 grown in a nutrient solution to which 

 neutral red has been added, made it 

 possible to record this phenomenon 

 under excellent conditions (Fig. 120). 

 In the extremities of growing filaments 

 the vacuoles generally appear as very 

 small elements shaped as granules, rods or filaments. These ele- 

 ments are carried along by cytoplasmic currants which cause them 

 to change shape constantly. They are capable of swelling and of 

 contracting, of passing from the shape of granules to that of fila- 

 ments and conversely. In the space of a few seconds, they are 

 frequently seen to fuse and form rather large spherical vacuoles 

 which, themselves, may give rise, by budding, to small vacuoles, or 

 may be completely split up into a multitude of small elements which 



Fig. 122. — Drosophyllum 

 lusitanicum. Epidermal cells 

 of the pedicel of a tentacle. 

 Vacuoles in grey, arrows in- 

 dicate the direction of the 

 head of the gland. A, during 

 digestion of protein. B, dur- 

 ing inactive period. C, tip 

 of leaf with tentacles; one 

 gland showing pedicel, head 

 and drop of secretion. (Af- 

 ter Mangenot). 



