Response to Stimains.—Irritability is a funda mental property of protoplasm, so we Mid it present ill unicellular organisms and in the tissue cells. Protoplasm is active only within certain limits of temperature, tiepin(' which quiescence—heat rigor or cold rigor —occurs. Protoplasm (of 1:Nlyxotnycetes) will tlow with reference to a source of heat —toward that tem perature to %Odell it is most `attuned.' I'he direc tion of light-rays may also determine the direc tion of Tocomotion. When a ray of white light passes obliquely through an anneba, the latter moves from the stamp of light. AIM), the chloro phyl bodies of a leaf will move away from the surface which is too intensely illuminated. Like wise the electric current, gravity, contact, and ehemieal agents may awaken definite responses in cell-protoplasm.
The forms assumed by cells are most diverse. In the animal's body they are united to form tissues, and the cells of each kind of tissue have their peculiar form. The cells of the skin are flattened or enboidal: those lining the food-canal are cdhunnar: muscles are composed of elongat ed compound cells and nerves give rise to proc esses that may be two feet or more in length. These kinds of cells will etaisidered under llisrotonv.
77(c (km-0•7.-111e egg and the spermato zoon, the uni Of which is the initial act in sexual reproduction, arc each single cells, de rived, like the other cells of the body, by the division of preexisting cells. The two kinds of germ-cells are, however, very different in ap pearance and function. The (ovum) ii of great size. due to the fact that it is stuffed full of food-material, the so-called yolk. The nucleus is large, and is commonly called the germinative vesicle, and in it- open-meshed films the chromatic material is lodged. The entire egg is often enveloped in one or more envelopei tor its protection. 'the .pennatozoiin is of ex tremely small size. It is typically a thread. like cell, enlarged at one end to form the head. Behind the head is the middle piece, followed by the lash or tail. The head contains the nucleus, which is here dense, without water. but consist
im• almost wholly of chromatin. The middle piece seems to contain the centrosome. The outer envelope of head and middle piece and the whole of the tail are eytoplasmie. and play only an incidental part in the fertilization of the cell. Set` FERTTLIzATTON.
E. B. Wilson, The Cell in Derelop ment and Inheritance, page 4`.:3 (2d ed., New York, 1 900) t 0. Hertwig, Die %elle tend die acwebe (Jena. T., I393: 1 I., 1899: Eng. trans., New York, 1911I1), CELL (in plants). Tn its most fundamental characters the plant-cell closely resembles the animal cell. Like that, it possesses a nu Melts and cytoplasm, and may or may not he surrounded by a cell-wall. Nearly all cells arc microscopic in size, a fair average bring about 0.004 to 0.002 inch in diameter. Some cells, however. become much elongated, bast cells ovpr 0.2 in length having been reported, and the internodal yells of Chaim sometimes reaching a length of 2 inches. on the other hand, the cells of the yeast-plant are only 0.00032 inch in diameter, and the bacteria are much smaller. When cells are free, is a tendeney to assume the spherical form. and this is the most usual shape in one-celled plants: but where the cells are aggregated into tissues vari on- modifications are met. the cells becoming compressed. elongated, flattened. star-shaped, etc. In all except a few of the lowest plants, the cell has a wall of cellulose. 8 snlistanee de rived from the A, growth pro ceeds. the original mall of cellulose may become modified, the various change, being called ligni fication, suberization. etc. In spores the outer portion of the wall usually develops spines, fur rows. and various sculpturing... that are suffi cientl• constant to be of value in classification. The internal modification. of the cell wall are no less numerous and striking. Sometime, there is a uniform thickening, hut more commonly the thickening is irregular and results in the ap pearance Of striations, spirals. rings, reticula tions. and different of pits.