286 



METAMORPHOSIS 



We will commence by studying a member of the family of liverworts 

 (AsKENASY, 1874). In Pellia epiphylla the fertilized egg-cell gives rise after 

 several months to a sporogonium, of which only the median region, the seta, 

 is of interest to us at present. The young seta consists of numerous embryonic 

 cells, but even after several months growth is only 2 mm. long. When the 

 spores begin to ripen, however, it begins to grow rapidly, reaching a length 

 of 80 mm. in three or four days, thus elevating the 

 capsule into the air. The cells of the seta are at first 

 filled with protoplasm and contain abundant starch, 

 but after this great elongation has occurred, the 

 starch is found to have been completely used up (having 

 l>een employed in the manufacture of new cell-walls), 

 and the protoplasm now forms merely a thin layer in 

 the interior of each cell, which is occupied chiefly by 

 large vacuoles. It is especially interesting to note 

 that no cell division occurs during the elongation. 

 This elongation does not take place equally and uni- 

 formly throughout the entire length of the seta, but by 

 differentiation of a zone of maximum growth, which, 

 however, does not remain constantly in one place. 



Similar growth elongations are known to occur 

 among other lowly-organized plants such as certain of 

 the larger Fungi and many Algae, such as the unicellular 

 Siphonaceae. These appearances are met with, how- 

 ever, especially among the higher plants, and to these 

 we may now direct our attention. The chief features 

 of this elongation are as follows : — 



1. Growth at the very beginning is accompanied 

 by cell division, but later on this division ceases, and 

 the cells elongate considerably. 



2. The increase (Fig. 82) in the volume of the cells 

 is by no means equalled by the increase in the amount 

 of the protoplasm, hence the vacuoles become larger 

 and larger till they reach a quite remarkable size, 

 apparently by absorption of water. In this way 

 growth may be effected rapidly and at little cost 

 of material. 



3. Growth is not uniform, but varies in amount 

 in a manner still to be described. 



Various methods are employed for measuring 

 growth ; we will describe those first which aim at 

 determining the total growth in length of an organ. 

 For rough measurements an ordinary scale is suffi- 

 cient ; for more accurate measurements, especially 

 when these have to be taken successively at short 

 intervals, we employ a reading microscope. In dealing 

 with vertically growing structures such as roots and 

 shoots the tube of the microscope must be placed 

 horizontally, and for this purpose Pfeffer's horizontal microscope will be 

 found very appropriate. In addition to the optical method of magnifying the 

 amount of growth we may employ mechanical means, e. g. a lever. A very 

 simple form of ' auxanometer ' is that used by Sachs, which consists of arc 

 and pointer (Fig. 83). A light wire frame carries a small pulley [r) and a long 

 pointer {z) playing on a scale {q). If a fine silk thread be attached to the apex 

 of the plant and then carried over the pulley and kept stretched by a small 



Fig. 82. Elongation of cells 

 accompanied by increase in the 

 size of the vacuoles (x 500). 

 From the Bonn Textbook. 



