THE GROWING POINT 



273 



H'N'I-) 



Fig. 62. Diagram of a cell filament with 

 an apical cell, s. 



logically alike ; on the contrary, we meet with greater and greater differences 

 between the individual cells as we ascend in the organic scale, and these differ- 

 ences are indicative of their physiological activities. Every cell has no longer 

 the same function ; division of labour has appeared, and," as a consequence, 

 the individual cells have lost their physiological individuality and have 

 become mutually dependent on each other ; in a word, they are no longer 

 capable of individual and isolated existence and can carry out their functions 

 only when united into a ' differentiated ' com- 

 plex. The first of these differentiations is the 

 separation of a series of cells capable of con- 

 tinued growth and division from cells which, 

 having reached the adult state, are in a resting 

 condition, and since these merismatic cells are 

 not distributed irregularly among the resting 

 cells, but occur in the simplest case at one end of the plant body, it comes about 

 that this primary differentiation results in the formation in the plant of two 

 poles, a base and an apex. Let us study, as a simple example, a cell filament, 

 composed of a row of cylindrical cells, a, b, c, &c., all fully developed. The 

 apex is occupied by the only cell capable of further growth (the apical cell s), 

 differing in form from the rest. When the apical cell has reached a certain length 

 it divides, and the daughter - cell 

 d, lying next to c, becomes a resting 

 cell while the other, s', remains as an 

 apical cell, and so the process goes 

 on indefinitely. We may term the 

 apical cells embryonic cells, and the 

 cells derived from them somatic 

 cells. Hence we may describe the 

 higher plants which exhibit this dif- 

 ferentiation as somatophytes, whilst 

 the lower forms, which possess no 

 fully developed region, i.e. no 

 'soma', may be termed asomato- 

 phytes (PFEFFER, Phys. II, 2). 

 Although the transition from em- 

 bryonic to somatic, or fully deve- 

 loped, cells takes place gradually, 

 still the contrast between them is 

 sufficiently distinct. It will be better 

 to speak in this case not of cells but 

 of 'substance', embryonic substance 

 and somatic substance, for the em- 

 bryonic substance need not be con- 

 fined to one cell, as in the example 

 we have taken, but may exist in 

 many cells or may be limited to 

 a part of one cell. The regions where 

 the embryonic substance is found in a plant may be styled, following the older 

 nomenclature, the 'growing points'. The growing point, however, does not 

 always lie at the free apex of a plant body ; it may occur at the base or between 

 two somatic regions. We have, therefore, to distinguish terminal, basal, and 

 intercalary growing points. 



In the simplest case the entire activity of the growing point is devoted 

 to the elongation of a previously existing body, but when the plant is branched 

 then the growing point has also to provide for the production of these outgrowths. 



Fig. 63. Dichotomy in the growing point of Dictypta. 

 a, apical cell (x 500). After DE WILDEMAN in the Bonn 

 Textbook. 



JOST 



