296 METAMORPHOSIS 



activity of the growing point of the Siphonaceae was quite independent of the 

 presence there of a large amount of protoplasm. [Further, Noll (1903) has also 

 shown that the protoplasm in the apex of the growing point does not differ 

 from the remainder, that it overflows into it and is supplied from it.] He also 

 showed that in numberless lower plants every cell is in an ' embryonic ' condi- 

 tion, and yet that the protoplasm is present there in only moderate quantity. 

 The ordinary condition of the cells found at the growing point must be explained 

 otherwise. Pfeffer (Phys. II, p. 7) points out that the abundance of proto- 

 plasm may be accounted for by assuming that it is intended to permit of rapid 

 growth in length by the taking in of water, without any further construction 

 of protoplasm. 



From the cells of the growing point are derived all the permanent tissue- 

 elements in the higher plant, no matter how varied they be in appearance and 

 in function. Their differentiation takes place at different times in the several 

 organs. While, for example, the definite structure may be mapped out, though 

 not completed, in an internode still undergoing elongation, anatomical dif- 

 ferentiation takes place in a root at a later stage, often long after elongation has 

 ceased. Certain elements, such, for example, as sclerotic cells, which have no 

 longer the power of growth, may expand after the completion of their legitimate 

 length, while, on the other hand, vessels generally push ahead of all other ele- 

 ments. Their early appearance is obviously necessary because the demand 

 made on water by the growing points can only be supplied by a continuous 

 water channel. 



To trace the transition of embryonic cells into permanent tissue-elements 

 in detail would necessitate a study of the fundamentals of plant anatomy ; we 

 must limit ourselves, therefore, to a consideration of principles only and 

 refer for details to the special literature on the subject, especially Haberlandt 

 (1896). 



Let us first look at the alterations in the general outline of the cells. As 

 they are approximately isodiametric at the growing point, they must become 

 pulled out during longitudinal growth unless their original length is reproduced 

 by continuous transverse division. The relative length, that is to say, the 

 relation of length to diameter, may be increased by several longitudinal divisions. 

 Very frequently there is a tendency on the part of cells to round themselves 

 off ; in this way walls meeting each other at an angle of 90° get displaced in 

 such a way that now three walls meet together at one point at an angle of 120°. 

 At the corners or angles also, owing to increased stretching, a splitting of the 

 middle lamella often takes place, in consequence of which intercellular air- 

 spaces appear, communicating with each other ; these spaces are of extreme 

 importance in relation to gaseous exchange. Possibly all these rounding-off 

 processes may be explained in the first instance by osmotic pressure in the 

 interior of the cell, in regard to which the cell-wall behaves passively. But 

 active local growth of the membrane is also an important factor in form dif- 

 ferentiation. Just as from epidermal, so occasionally from internal cells border- 

 ing on intercellular spaces outgrowths (hairs) may arise. The vessels may become 

 filled with ' thyloses *, ingrowths from neighbouring cells which become pressed 

 against each other, and so the vessels may become blocked up by a luxuriant 

 cellular growth entering through clefts formed in the course of development 

 (e. g. between separated sclerotic rings). Local surface growth may appear 

 also in individual cells in compact tissues, and these cells may force their way 

 between neighbouring elements, splitting their middle lamellae and sliding over 

 their ceU-walls. This kind of growth has been termed 'sliding growth', and 

 appears to be of much more general occurrence than was at first imagined. When 

 no individual cell grows more rapidly than its neighbour, but the whole tissue 

 shows equally vigorous surface growth, the resulting form is attained without 



