ELONGATION AND INTERNAL DIFFERENTIATION 295 



not become transformed into permanent tissue, but retains its embryonic 

 characters. This cambium is a remnant of the primary growing point. If it 

 alone be present then the bundle only increases in thickness ; but for the most 

 part there arises also an interfascicular cambium, i. e. certain cells of the medul- 

 lary rays, having all the characters of permanent tissue elements, revert to the 

 embryonic condition and form cambium arcs uniting themselves to the already 

 existing cambiums on either side. In this way arises in a cylindrical organ, 

 a complete circular (in transverse section) intercalary growing layer which 

 produces new tissue actively and continuously. Fascicular and interfascicular 

 cambiums possess the same capacities, so that it is immaterial whether the 

 origin is directly or indirectly from the growing point. Ordinary parenchyma- 

 tous-cells are simply intermediate stages between embryonic and permanent 

 tissue-elements, which retain for a long time, frequently all their lives, to 

 a certain degree, a capacity for renewal of growth, although they do not always 

 exercise it. Although we may make a sharp distinction between primary and 

 secondary growth in thickness, we do not do so with regard to growth in length ; 

 more careful consideration shows, however, that the intercalary growing regions, 

 such as those at the bases of leaves of Monocotyledons or in many internodes, 

 have as much right to be termed secondary growth areas as cambium has. We 

 may, therefore, speak perfectly legitimately of secondary growth in length. 



Before we leave the consideration of growth in length we must study 

 certain peculiar phenomena concerned with the relationships of growth in length 

 and in thickness. If longitudinal growth be rapid, a diminution in diameter may 

 take place, and vice versa. A diminution in diameter, although quite insignificant, 

 occurs, according to Askenasy (1879), in the stamens of Gramineae, which in 

 a quarter of an hour may increase their length fourfold by absorption of water. 

 The converse process occurs much more frequently ; it was found by Berthold 

 (1882) in Antithamnion, and is of common occurrence in roots. In the latter 

 (De Vries, 1880 ; RiMBACH, 1897), immediately after vigorous elongation, an 

 increase in thickness occurs which causes a reduction in length of as much as 

 10-70 per cent. This reduction is due to a certain alteration in form taking place 

 in some but not all cells. Owing to the activity of these cells, an activity not 

 as yet sufficiently well understood, other tissues, such as those of the cortex 

 and of the vascular system, which are unable to contract, are thrown into folds. 

 This contraction in the root is of very great importance. Its effect is to draw 

 down more and more towards the soil the leafy regions of many * rosette plants ', 

 in spite of the continued elongation of the axis by growth ; it causes and regu- 

 lates the entry of many tubers and bulbs into the soil to a definite depth, and 

 finally strengthens the hold the plant has on the soil, because tense roots render 

 the plant as a whole more stable than slack ones. 



Following Sachs, we have distinguished three periods of growth ; during 

 the first of these the different members are laid down on a certain plan, during 

 the second their absolute and relative size is determined, during the third 

 (which we have now to deal with) the internal anatomy is developed. These 

 three periods, as already noted, cannot be sharply marked off from each other, 

 more especially in the case of the third, which often begins before the first has 

 come to an end. 



When the growing point is multicellular, its cells are generally full of 

 protoplasm ; each possesses a large nucleus and shows no vacuoles. The growing 

 point in such plants as the Siphonaceae or Mucorinae, which consists merely of 

 a part of one cell, also shows, for the most part, a dense aggregation of proto- 

 plasm in that region. We might, therefore, conclude that abundance of 

 protoplasm is one of the most general characteristics of an embryonic cell and 

 closely bound up with its specific function. A number of facts, however, tend 

 to show that this conception is incorrect. Thus Noll (1902) found that the 



