THE VITAL PROPERTIES OF THE CELL 171 



according to a law of its own. Many plant cells grow only at 

 one point : either at the apex or near the base, or they develop 

 lateral outgrowths and branches. Others undergo during growth 

 complicated changes of direction, as in the internodes of the 

 Characece. 



Finally, Nageli states, as a point in favour of the theory of 

 growth by intussusception, that many membranes increase con- 

 siderably both superficially and in thickness after they have 

 become separated from the protoplasmic body, in consequence of 

 the formation of special membranes around the daughter-cells ; 

 " Gloeocapsa and Gloeocystis appear first as simple cells with a thick 

 gelatinous cell-wall. The cell divides into two, whereupon each 

 develops for itself a similar enclosing cell-wall, and in this manner 

 the enveloping process proceeds." The outermost gelatinous cell- 

 wall must in consequence become larger and larger. According 

 to Nageli's computation, their volume during successive develop- 

 mental stages may increase from 830 cubic micromillimetres to 

 2,442, to 5,615, and finally to 10,209 cubic micromillimetres. 



In another species the gelatinous cell-wall was seen to increase 

 from 10 to 60 micromillimetres, that is to say, it became six times 

 as thick. " In Apiocystis the pear-shaped colonies, which consist 

 of cells embedded in a very soft gelatinous matrix, are surrounded 

 by a thicker membrane. In this case, moreover, the membrane 

 increases with age, not only in circumference but also in thick- 

 ness ; for whilst in smaller colonies it is barely 3 micromilli- 

 metres thick, in larger ones it is 45 micromillimetres thick ; in 

 the former it is 27,000 square micromillimetres in area, and in the 

 latter 1,500,000 square micromillimetres. Thus the thickness of 

 the sheath increases at a ratio of 1 to 15, the superficial area of 

 1 to 56, and the cubic contents of 1 to 833. That apposition 

 should take place ujion the inner surface of this sheath is out of 

 the question, for its smooth internal surface never comes into con- 

 tact with the small spherical cells, or only does so in a few isolated 

 spots." 



In all these cases I am obliged to agree with Nageli, who con- 

 siders that we have to make too many improbable assumptions, if 

 we attempt to explain the superficial growth of the cell membrane 

 solely by the deposition of new layers, whereas the above-men- 

 tioned " phenomena {variations in form and direction, uneven growth 

 of various parts, torsions) may be explained in the simplest and 

 easiest fashion by intussusception. Everything depends upon this, 



