DICOTYLEDONS. 



561 



period at the base of the primary stem which grow as strongly, and thus give rise to 

 a shrubby plant. When the axis of the embryo grows vigorously, the primary 

 root generally also grows vigorously in a downward direction^; and a Tap-root is 

 thus formed, from which, as long as it increases in length, the lateral roots spring in 

 great numbers in acropetal succession. When the growth in length of the tap-root 

 ceases, adventitious roots become intercalated among the lateral roots already 

 formed, and like them, grow vigorously, and may themselves produce lateral roots 

 of higher orders. A strong root-system is thus produced with the primary root of 

 the embryo for its centre, which endures as long as the stem itself. By the subse- 

 quent increase 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 

 these processes, which are here described in their main outlines, take place almost 

 invariably among Conifers, a number of deviations occur, on the other hand, 

 among Dicotyledons similar to those which have been spoken of under the head 

 of Monocotyledons. The primary axis may die soon after germination or at the 

 end of the first period of vegetation, the primary root often perishing as well, while 

 the axillary shoots of the cotyledons or of subsequent leaves continue the life of the 

 individual. Thus, for example, in the dahlia, a strong adventitious root is given out 

 laterally from the hy]iocotyledonary portion of the axis at the close of the first period 

 of vegetation of the young plant, and swells into a tuber ; the primary root-system 

 and the portion of the axis above the cotyledons disappear, and there remain only 

 for the continuance of the life of the plant the new tuberous root, the hypocoty- 

 ledonary portion of the axis, and the axillary buds of the cotyledons. The process 

 is still more striking in Ramuiculus Ficaria, where, after the development of the 

 primary root, a tuberous lateral root is produced below the primary axis of the 

 embryo, sheathed by a coleorhiza, and maintains its existence together with the 

 axis, while the primary root and the first leaves perish. Among the numerous 

 cases belonging to this category may be mentioned also Physalis Alkekengi, Mc7itha 

 arvensis, Bryonia alba, Polygonum amphibium, and Lysimachia vulgaris"-. The pro- 

 duction of bulbs also occurs among Dicotyledons (as in species of Oxalis), though 

 not so commonly as among IMonocotyledons ; of more common occurrence are 

 tubers or swellings of underground branches, stolons, or rhizomes of greater or less 

 thickness. The greater number of Dicotyledons have perennial underground roots 

 or stems which send up periodically leafy and flowering shoots that die at the end 

 of each period of vegetation. In all such cases, where the primary root-system 

 of the seedling perishes, new roots are repeatedly developed from the stem; and 

 the power possessed by most Dicotyledons of producing adventitious roots from the 



' One of the most remarkable exceptions is afforded by the genus Cuscuta, which has no 

 primary root, the posterior end of the axis penetrating into the ground on germination, but soon 

 dying off when the upper filiform portion of the stem has embraced the plant on which it becomes 

 parasitic, and has fixed itself on to it by its short suckers; the plant afterwards grows vigorously 



and branches. 



2 The above is taken from Irmisch's detailed descriptions in his Beitrage zur vergleichenden 

 Morphologic der Pflanzen, Halle, 1854, 1S56 ; Bot. Zcltg. i85i ; and elsewhere. 



o 



