MORPHOLOGY OF THE CELL 33 



upon surface tension, certain substances tend to collect more at the sur- 

 face than do other things, thus making the composition of the surface 

 layer differ from that of the deeper-lj'ing protoplasm. Surface tension 

 also tends to force the larger globules contained in the protoplasm (vide 

 infra) away from the surface, leaving only the smaller ones in the periph- 

 eral layer. For these reasons, a very thin surface layer of protoplasm 

 has a different composition and consistency, and is often known as the 

 cell membrane (Fig. 20, cm) ; but this membrane is not a secretion, it is 

 part of the living matter itself. The term cell membrane is used, how- 

 ever, for other structures some of which may be lifeless. 



Plastids are not uncommon structures in cells (Fig. 20, p). In the 

 higher plants they are universal, and they are usually green, though 

 some are of other colors, giving color to certain fruits and flowers. Some- 

 times they are colorless, as in the embryos of green plants and in certain 

 parts of the adults as well. In animals, plastids are not uncommon in 

 certain classes of Protozoa, where they are mostly colored. Each kind 

 of plastid serves a different function, one of which is discussed in another 

 chapter. 



Vacuoles are vesicles of liquid enclosed in the protoplasm. They 

 may be permanent or temporary. In the Protozoa, temporary vacuoles 

 are common. They usually either enclose bodies of food in process of 

 digestion, in which case they are called food vacuoles, or they disappear 

 at intervals by ejecting their liquid contents through the surface layer 

 of protoplasm into the surrounding medium. After such ejection, a 

 vacuole is formed again, often at the same point, only to disappear 

 later. Such a vacuole is called a pulsating or contractile vacuole. (See 

 Fig. 20, V.) 



In some cells an attraction-sphere or centrosphere (Fig. 20, a^is found, 

 usually near the nucleus. It is a mass of somewhat differentiated pro- 

 toplasm, containing a minute body that stains deeply, the centrosome. 

 When present the centrosome takes a conspicuous though probably not 

 important part in cell division, as described in another chapter. 



Besides all the above structures which serve, or appear to serve, some 

 function in the cell, and which may therefore be regarded as cell organs, 

 there are often lifeless matters enclosed in the protoplasm. These may 

 be grains of starch or related substances, or oil or fat globules, which 

 the cell has produced and which are stored as future food. Again, these 

 lifeless objects may be undigested remains (such as shells) of organisms 

 taken as food, or even objects picked up incidentally along with food or 

 otherwise. When these non-living objects, which may be spoken of as 

 cell inclusions (ci), are wholly foreign in their origin, they are not to be 

 regarded as parts of the cell. When, however, they have been produced 

 by the activities of the cell itself, as starch grains or oil globules, there is 

 justification for regarding them as true components of the cell. 



