18 



PHYSIOLOGY 



CHAP. 



that the reticulum existed in appearance only, and was merely the 

 optical expression of the finest vesicles in close apposition. Pro- 

 toplasm thus consists of a foam-like ground-substance, constructed 



FK-. 7. Alveolar structure of protoplasm. (Biitschli.) a, Delicate foam of alveolar structure 

 obtained by prolonged whipping of olive oil and cane-sugar; l>, alveolar structure of intra- 

 capsular protoplasm from Thalassicolla nudeata, as in Fig. 2. 



in the form of delicate polyhedric vesicles or alveoli, closely pressed 

 together. The protoplasmic granules lie in greater or less num- 

 bers at the corners of the foam-bubbles, never in the liquid of the 

 alveoli themselves (Fig. 7). 



Even under the low power, apparently 

 homogeneous protoplasm not infrequently 

 exhibits drops of fluid, or vacuoles, as they 

 are somewhat infelicitously termed. Such 

 accidental vacuoles must be distinguished 

 from the permanent ones, which are so 

 numerous and conspicuous in certain plant 

 cells as to give a spongy appearance to the 

 protoplasm (Fig. 8). Rhythmically pulsating 

 vacuoles may sometimes be observed ; these 

 empty themselves on contracting, and refill 

 with fluid on dilating. This is especially the 

 case in certain kinds of Amoebae, and is very 

 frequent among the ciliated Infusoria. In 

 these cases the vacuoles function as a centre 

 of circulation for the protoplasmic fluid. 



Besides the vacuoles, there are in vege- 

 table protoplasm granules of chlorophyll, 

 FIG. s. -ceii from staminai starch, and aleuron : in animal protoplasm, 

 & ^s^buTgero f ^ globules, accumulations of glycogen, and 

 The nucleus is surrounded granules known as " vitellin." The chloro- 



by a spongy network with i n , 



wide meshes. P n 7^ corpuscles are oi capital importance 



to the plant cell, since the most characteristic 



part of its vital processes depends on them ; viz. the reduction of 

 carbonic acid, and fixation of carbon. The granules of starch, 

 aleuron, fat, glycogen, and vitellin are nutritive materials, products 

 of protoplasmic activity, stored up within the cell. 



