PROTOPLASM 47 



portion of the fibers of the achromatic figure at the close of mitosis. His 

 observations were made on the endosperm of Lilium and Tamus. Others, 

 on the contrary, have regarded them as secondarily developed structures. 

 Their absence from the walls between Cuscuta and Viscum and their 

 hosts (Kienitz-Gerloff, Kuhla, Strasburger 1901), and also from many 

 cells which glide over one another during growth, is a fact opposed to the 

 latter interpretation. Although they have been demonstrated in a 

 number of kinds of tissue they probably do not occur so widely as some 

 have supposed; but it may nevertheless be true that in many cases their 

 apparent absence is due to the fact that the special methods often neces- 

 sary to their demonstration have not been widely employed. 



As to their function, it can scarcely be doubted that they may serve 

 to transmit stimuli of one kind or another from cell to cell (Pf eff er 1896) . 

 Noteworthy in this connection is their presence in tissues of plant parts 

 known to be particularly responsive to external stimuli, such as the leaves 

 of Mimosa (Gardiner 1884) and Dioncea (Gardiner 1884; Macfarlane 

 1892), the stamens of Berberis (Gardiner 1884), and the sensitive labellum 

 of the orchid, Masderallia muscosa (Oliver 1888). Their extensive 

 development in storage tissues, such as the endosperm of seeds (Tangl 

 1879; Gardiner 1897), would also indicate that they are in part responsible 

 for the readiness with which nutritive materials are translocated in such 

 specialized tissues. 



Vacuoles. Vacuoles in the cytoplasm are more characteristic of 

 plant than of animal cells. They are usually absent in the very young 

 cell, but appear as growth and differentiation progress. In case they are 

 very small and numerous the cytoplasm takes on an alveolar appearance, 

 but more commonly they coalesce to form one large vacuole which 

 may occupy a volume greater than that of the protoplast itself (Fig. 2). 

 This condition is characteristic of many mature cells of plants, but is 

 comparatively rare in animals. 



The ordinary vacuole is essentially a droplet of fluid, consisting of 

 water with differentiation products in solution, surrounded by a delicate 

 limiting membrane. DeVries (1885) developed the theory that vacuoles 

 are derived from "tonoplasts." The tonoplasts were believed to be 

 small bodies imbedded in the cytoplasm and multiplying by fission. 

 Through the absorption of water they swell and become vacuoles, the 

 vacuole wall thus being made up of the material of the tonoplast body. 

 We still refer to the vacuole wall as the tonoplast. De Vries looked upon 

 the vacuole as a body with an individuality somewhat similar to that 

 of a nucleus, since the tonoplast from which it develops was supposed to 

 arise from a preexisting tonoplast by division. The theory was supported 

 by certain other workers, but it does not enjoy wide acceptance today 



It has been found by Bensley (1910) and others that there is in the 

 cytoplasm of certain comparatively young cells a system of fine canals 



