78 MERISTEMATIC SYSTEM 



primordial meristeni into a safe position, is the involution of the apex 

 which is seen in certain Seaweeds (spp. of Polysiphonia and Hdico- 

 thamnion). An effective protection of the growing-point is most 

 urgently required in the case of structures which pass the whole of 

 their life in the soil and thus carry out their entire development 

 within a solid medium. A typical illustration is provided by ordinary 

 roots. ' Here, moreover, owing to the absence of appendages which 

 might lend themselves to the formation of a bud, the growing-point 

 is, as it were, thrown back upon its own resources for the means of 

 protection. As a matter of fact the meristeni solves this problem 

 very successfully by producing a resistant root-cap ; it consequently 

 appears to occupy an internal position in comparison with the obviously 

 superficial primordial meristeni of the shoot. 



II. THE PRIMORDIAL MEREST EM. 



Inasmuch as the primordial meristeni provides all the structural 

 elements out of which stem, leaf and root are built up, it represents 

 in the most literal sense the original or primordial formative tissue of 

 the plant. In accordance with the universal nature of its relations, 

 the primordial meristeni exhibits no differentiation which could be 

 regarded in the light of a preparation for the development of particular 

 anatomico-physiological tissue-systems. Any inequalities of size, shape 

 or arrangement which its component cells actually display, depend 

 entirely upon the manner in which successive tissue-elements are 

 segregated from the meristematic cells. "While the sequence of the 

 cell-divisions accompanying this process of segmentation is sometimes 

 very regular and obvious, this is by no means always the case. Since 

 Nageli's classical researches upon the subject, 35 much time and labour 

 have been expended by a number of botanists upon its further 

 elucidation. 



A. MARGINAL AND APICAL [MERISTEMATIC] CELLS/* 



The distribution and arrangement of the meristematic tissues in 

 the plant-body, or in a particular organ, depend upon the shape and 

 mode of growth of the structure under consideration. Thus in the 

 case of a more or less circular plate of cells, the marginal elements, 

 which are all homologous and endowed with an equal capacity for 

 growth, collectively represent the primordial meristem. These marginal 

 cells divide by walls parallel to the margin of the plate, and thus 

 gradually advance in a centrifugal direction : in this way they soon 

 become the terminal members of cellular filaments, which radiate 

 outwards like the leaves of a fan. The thallus of Melobesia (Fig. 9) 



