THE CELL 33 



Jensen and a number of others have shown that a stimulus may- 

 be transmitted down a growing root and through an intervening 

 layer of gelatin after the tip of the root has been cut off and 

 replaced, with gelatin between it and the root proper. The upper 

 part of the root is subjected to a light stimulus, and the lower 

 part on the other side of the gelatin layer responds by curved 

 growth. The tip of a foreign root may be attached (an oak root 

 cemented on with gelatin to a decapitated wheat root), and 

 response obtained. (Excitation is not transmitted through an 

 intervening layer of mica or tinfoil.) The stimulus cannot be 

 vital in the strict sense. It may be electrical, or a diffusion 

 wave, traveling in a salt solution. Such experiments are inter- 

 esting and instructive, but they do not disprove the hypothesis 

 that while some vital processes, such as that of stimulus trans- 

 mission, may be quite simple physical ones, others are of such 

 complexity that they can take place only in living protoplasm. 

 It is our task to distinguish the one from the other. 



Cell Types. — The cells that we have so far discussed are among 

 the usual ones of tissues, but there are many other types. Some 

 unique ones occur as parts of tissues, and other extraordinary 

 ones are complete organisms in themselves. Of the unique types 

 of tissue cells the following are a few. Root hairs, so important 

 in the life of plants, are elongated epidermal cells. On the 

 surface of the leaf of the nettle are slender hairs consisting of a 

 single cell filled with a poisonous sap. The wall is thin near the 

 tip, so that if the hair is pressed upon, it breaks, leaving a sharp 

 point. The cell thus acts Hke a hypodermic needle. In the 

 pith of the rush Juncus, the cells are starlike, providing abundant 

 air space. Thus does the shape of a cell suit the particular func- 

 tion to be performed. 



Plant cells having specially developed walls are common. The 

 tracheids, which carry water in the stem of the pine, have 

 communicating bordered pits in their walls. Cells of the epi- 

 dermis of leaves develop an enormously thick outer wall impreg- 

 nated with waxy cutin. "Stone" cells with remarkably heavy 

 walls are common in plant tissues. The cells in the seed of the 

 date store food in their walls for the young seedling, which later 

 digests it when the seed germinates. 



Cells in the animal body may be as varied. The cells lining 

 air passages are covered with cilia, or little hairlike whips. Nerve 



