AND IRRITABILITY OF SENSITIVE PLANTS 189 



similar to nerves. Haberlandt considered the carriers of stimuli to be 

 special cells that are found only in sensitive plants. He described these 

 cells as being 6 to 4.2 mm. long and of an average width of .18 mm. 

 The walls of these cells are thin and are dotted; the dividing end walls 

 are finely porous and penetrated by plasma threads. The cell contents 

 consist of a thin plasmatic peripheral layer with a very large, round, or 

 elongated nucleus. The contents of each cell contain mucilage, glu- 

 coside and resin. These cells, according to Haberlandt, are interwoven 

 with the vascular bundles of stem, leaf, petiole and pulvini. The whole 

 conducting system acts as one in which intercommunication exists, and 

 one that is fused and through which hydrostatic waves pass and propa- 

 gate stimuli. 



Borzi (4) emphasized certain structural details of various sensitives — 

 species of Mimosa, Aeschynomene, Neptunia — as aiding in the explana- 

 tion of sensitive relations. He described (1) the structure of the peri- 

 pheral regions — epidermis, hairs, etc. — as the parts that perceive sti- 

 muli directly; (2) The deeper conducting regions; (3) The pulvini as 

 the irrito-contractile centers. 



Mac Dougal (27), in his studies on Mimosa pudica, showed that 

 Haberlandt's explanation of transmission of stimuli by the special cells 

 of the phloem was not tenable. Stems from which the phloem region 

 was removed were still able to transmit stimuli. "Excluding the 

 hydrostatic theory of Haberlandt, at present it seems necessary to 

 assume transmission by the tissues of the entire cross section." 



Mac Dougal (28, 29) also studied the propagation of stimuli in 

 Biophytum sensitiviim. In this work he came to the conclusion that 

 the path of the transmission of the stimuli is the parenchyma of the 

 fibrovascular bundles and that the impulse is conducted plasmatically. 

 He says "it seems quite possible that the protoplasm action plays a 

 part in carrying the impulse from the point of reception to the motor 

 organ, and that while hydrostatic disturbance does not constitute an 

 impulse it may play a minor part in the transmission." 



Nemec (36) concluded that stimuli are propagated by the plasma 

 membranes of the cells, and by the intercellular connecting fibrils which 

 come in contact with the plasma membranes. 



Macfarlane (30) observed the flush changes in the tertiary pulvini 

 of various sensitives, such as Mimosa pudica, M. lupulina, M. sensitiva, 

 Schrankia angustata. "After stimulation of a leaflet and toward the 

 close of the latent period a sudden flush travels centrifugally across the 

 surface of the pulvinus. Immediately thereafter the leaflet contracts 



