The Substance Which Is Alive in Plants 153 



permits only a fixed position, which, however, is sufficient, since 

 the materials for making their food are brought to them in the 

 general circulation of nature. And these conclusions are all the 

 more confirmed by the seeming exceptions, for some plants swim 

 or creep freely about (e. g., swimming spores of Algae and Slime 

 Molds) in a very animal-like manner; but in these cases they lack 

 the firm cellulose wall distinctive of plants. But although the 

 skeletons of animals and plants differ, their protoplasm does not, 

 for in all essentials the protoplasm of plants and animals is alike. 

 This brief account of the plant skeleton has touched incidentally 

 on a matter which must now receive some further attention. As 

 the student soon learns when he studies many cells with his 

 microscope, they differ immensely in shape and in the thickness 

 and composition of their walls, to such a degree indeed as to 

 make them apparently too complex for analysis. Yet here, as 

 elsewhere, further study gradually crystallizes out the essentials, 

 when it appears that after all only a few ground forms exist, and 

 then only in correlation with definite functions or influences; 

 while all of the others are simply variations and combinations of 

 these. As to the shapes of cells, the simplest of all, and the one 

 to which all others tend to revert, is the sphere, that being the 

 mathematical form in which the most contents can be comprised 

 within the least wall. This shape, with the wall a spherical shell, 

 is actually realized in those cells which float freely in water or 

 air, as do the spores of many Algae and Molds, and some pollen 

 grains; and this shape may become elongated to ellipsoid and ovoid 

 forms under particular conditions (figure 49, 94, 108). Where such 

 cells occur inside the tissues of plants, however, and hence are 

 hard pressed by their numerous neighbors, the spherical shape 

 becomes necessarily modified to many-sided (polyhedral) or 

 faceted; and this shape is approximately realized in many stor- 

 age tissues of plants, where it comes measurably near to that 

 twelve-faced shape which always results when equal-sized spheres 

 are forced together by pressure (figure 49, 72). There is also some 



