134 GERMINATION, GROWTH, TISSUE TENSION. 



intervals be taken out, sterilised by being boiled in water, 

 then rinsed in water and replaced in the box. These 

 germination boxes will also be useful for various other 

 purposes e.g. experiments on geotropism. 



171. Broad Bean Seedling. Examine an entire well- 

 grown seedling, at least a foot in total length. Note the 

 root which has grown downwards from the seed, and the 

 shoot which has grown upwards from the seed. 



(a) In the root note (1) the main root axis, gradually tapering 

 to the free end or root tip ; (2) the rootlets, arising from the main 

 root in regular longitudinal rows usually five in Broad Bean and 

 differing from it only in their smaller diameter and different direc- 

 tion of growth ; (3) root-hairs well seen in seedlings grown in 

 moist air in germination jars or lamp-glasses ; (4) root-tubercles, 

 often seen in seedlings grown in soil. 



(6) In the shoot note (1) the axis or stem, four-sided and 

 hollow ; (2) the leaves, in two rows corresponding to two opposite 

 ridges of the stem ; (3) the buds, which in a well-grown plant may 

 have grown out as lateral branches, each bud or branch arising in 

 the axil of a leaf. 



(c) In a leaf from the upper part of the shoot note (1) the 

 petiole or leaf-stalk, grooved above ; (2) the stipules, a pair of 

 outgrowths at the base of the petiole, each like half of a spear-head 

 in form and having near the centre a dark spot this consists of 

 minute gland-hairs in a patch on underside of stipule ; (3) the 

 leaflets, thin flat oval appendages with a pointed tip ; (4) the 

 prolongation of the petiole above the leaflets this outgrowth, 

 sometimes developed as a small terminal leaflet, is evidently a 

 rudimentary tendril, as may be inferred by comparison with the 

 tendril- bearing Vetches and Peas related to Broad Bean. 



(d) Trace the root upwards and the shoot downwards 

 to their junction with the two large cotyledons or " seed- 

 leaves," which lie within the ruptured seed-coat. The 

 lower foliage-leaves are simpler in form than the upper 

 ones ; the two lowest (first formed) leaves above the 

 cotyledons are rudimentary and consist of three lobes 

 joined at the base. 



Also examine younger seedlings, working back to the 

 earliest stages in germination. Note that in the axil of 

 each cotyledon there is a bud; hence the cotyledons are 

 morphologically leaves, though in this plant differing 

 markedly from ordinary (foliage) leaves. 



