VARYING GROWTH IN LATERAL MEMBERS. 321 



this part of the bud-section, just as the rounding off of the 

 members from their original contact may also open up another 

 set of contact-parastichies. 



Again, since the transverse section must pass through the 

 insertion of the youngest members, the development of the 

 sheathing basal portion as a dorsiventral lamina will often form 

 the most conspicuous feature. Nymphaea alba (fig. 94) may be 

 taken as a type : the construction system for the seedling (2 + 3) is 

 identical with that of Sempervivum calcarafum (fig. 108), and in the 

 older members is closely imitated by the theoretical construction 

 (c/. fig. 110). The rhombs retain their original position, rounding 

 off somewhat in the petiole, and the dorsiventral lamina sprouts 

 on the younger members and slides among the older ones, cutting 

 off the original lateral contact-relations. Such a figure is, again, 

 practically identical with the bud-section of Banuiiculus repens 

 (perennating axis), in which the older leaves become compound, 

 but each remains packed within its own area ; also with that of 

 Isoetes lacustris, in which the sheathing effect is somewhat greater 

 and the contact much closer ; and finally, by complete fusion of the 

 sheathing portion of each leaf around the axis, the construction 

 becomes that of the apex of Bheum undulatum. 



To see the general effect of the diminution in the rate of 

 tangential growth of lateral members, unmodified by other agencies, 

 a shoot will be required which either bears uniform members in 

 large numbers, or in virtue of a slow rate of elongation in the 

 axis will admit of a large number of members being cut in one 

 transverse section. An example of such a type is found in the 

 rosette of Sempervivum ; the leaves are uniformly dorsiventral, 

 with no distinction of midrib and lamina, and are retained in an 

 unmodified bud-type of growth. The seedling of Pinus should 

 also afford suitable material, as also species of Sedum. The 

 general result will be that, if the rate of growth diminishes tan- 

 gentially, the apparent phyllotaxis system will be raised ; while if 

 the tangential rate be increased, the apparent construction system 

 will present a lower ratio system than the one actually laid down 

 at the apex. 



Sempervivum tectorum (figs. 2, 3). — The succulent leaves expand 



