450 ; '■ FOURTH GROUP. — SEED-PLANTS. 



\ 

 draws the upper ei^d with the cotyledons hanging from it out of the seed, and raises 

 them above the ground ; then the axis straightens itself and the cotyledons expand 

 in the air, and the plumule now in an advanced state of development shoots up from 

 between them. The cotyledons thus brought into the light usually increase rapidly 

 and considerably in size and form the fir^t green leaves of the plant, which are of 

 simple form in Cucurbita, the Cruciferae, Acer, the Convolvulaceae, Euphorbiaceae 

 and many others. If. the seed contains endosperm, the cotyledons are not with- 

 drawn from it, till the endosperm is exhausted (Figs. 375, 378). There are some 

 intermediate forms between the modes of germination which have now been described, 

 and special conditions of life sometimes lead to peculiar modifications ; in Trapa, for 

 example, the primary root, which from the first is rudimentary especially as regards 

 the root-cap, remains undeveloped, while the hypocotyl lengthens considerably and 

 turns its lower end upwards in the water at the bottom of which the seed has 

 germinated, and at an early period sends out rows of numerous lateral roots to fix 

 the plant in the ground. 



The increase in size of the young plant may take place by vigorous development 

 of the primary axis of the embryo ; as this grows (usually erect), the shoot which 

 developes from the plumule becomes the primary stem of the plant, and lengthens at 

 the summit and produces lateral shoots which are usually weaker than itself {Helian- 

 thus, Vicia, Populus, Impatietis, etc.) ; when the primary stem is persistent, it usually 

 sooner or later ceases to develope at its summit, or the lateral shoots nearest to the 

 summit become as strong as itself, and as the lower branches die off and leave the 

 stem bare the plant forms an arborescent head ; or the primary stem grows erect as 

 a sympodium {Tilia, Ricinus); or lateral shcots grow out at an early period from 

 the base of the primary stem, and develope as strongly as it, and a shrub is formed. 

 If the primary stem grows vigorously, the primary root of the embryo usually grows 

 also vigorously in the descending direction, and forms a tap-root, which sends out 

 numerous lateral roots in acropetal succession, so long as it continues to lengthen ; if 

 its growth in length ceases at some later time, adventitious roots are formed on it 

 between the former ones, and these also grow vigorously and produce lateral roots of 

 successive orders. In this way a large root-system is produced, the centre of which is the 

 primary root of the embryo, and it lasts as long as the stem itself. By subsequent 

 growth in thickness the primary stem as well as its branches assumes the form of a slender 

 upright cone, the base of which rests on the base of the inverted cone formed by the 

 primary root, which has also increased in thickness. While the mode of growth thus 

 simply described prevails almost without exception in ihe Coniferae, many deviations 

 occur in Dicoiyledons similar to those which have been already noticed in Monocoty- 

 ledons. The primary axis together with the primary root may die away soon after 

 germination or at the end of the first period of vegetation, while the shoots axillary to 

 the cotyledons or the higher leaves continue the life of the plant ; for instance a 

 strong lateral root is formed from the hypocotyl of Dahlia variabilis at the termina- 

 tion of the first vegetative period of the seedUng plant, and then enlarges and becomes 

 tuberous; the primary root-system and the epicotyledonary portion of the axis 

 disappear, and there remains only the new root, the hypocotyl and the buds axillary 

 to the cotyledons, to continue the vegetation. The arrangement is still more striking 

 in Ranunculus Ficaria, where after the development of the primary root a tuberous 



