j^O EXTERNAL CONFORMATION OF PLANTS. 



the formation of lateral members out of the punctum vegetationis occur in 

 nearly all plants, and by their regular repetition at definite points of the grow- 

 ing axis determine the external form of the plant, they may be considered 

 as normal, in opposition to the adventitious production of members which takes 

 place at the older parts of the axial structure at a distance from the apex and 

 without definite order. Such new formations are of equal importance in the 

 external form of the plant, and though adventitious are often of great import- 

 ance in a physiological point of view. Adventitious shoots are generally formed 

 in the interior near the fibro-vascalar bundles of the shoot, leaf, or root, and 

 are therefore endogenous ; but it does not follow from this that all endogenous 

 shoots are adventitious. All the shoots of Equisetum are endogenous in their 

 origin, but are not adventitious, since they are produced in the primary meristem 

 below the apex of the mother-shoot, and in a perfectly definite order. It is 

 equally incorrect to call all roots adventitious although they arise in the interior 

 of the stem leaves or roots. They are adventitious only when they occur in older 

 parts ; when they arise close to the growing point of a mother-root or a stem, 

 they are arranged in strictly acropetal order, and are for that reason not adven- 

 titious. When a member grows on a basal zone and produces lateral members 

 from it, they may be arranged in basipetal order, i. e. the younger a lateral member 

 is the nearer it wdll be to the base, as the sporangia on the columella of Hymeno- 

 phyllacese (according to Mettenius), or the lacinige of the leaves of Myriophyllum. 



(7) When in the higher plants a new individual is formed which is destined 

 for permanent and independent vegetation, a leaf-forming axis is first constituted, 

 that is, a shoot on which roots, hairs, and lateral shoots then arise. In all vascular 

 plants this first shoot (the primary stem) arises immediately out of the sexually pro- 

 duced embryo ; it appears therefore as if the externally unsegmented embryo is to 

 be considered as itself a primary shoot-axis ^ In Mosses, on the other hand, the 

 sexually produced embryo becomes transformed into the so-called Moss-fruit, a 

 structure without leaves, roots, or branches, the sole function of which is the 

 formation of spores. A new Moss-plant is, on the contrary, constituted by the 

 formation of a leaf-bearing shoot out of a branch of the alga-like Protonema, 

 which branches, strikes root (by root-hairs), and is independently nourished. The 

 shoot first produced, which developes the rest of the shoots and roots, is termed 

 the primary shoot, and its portion of the stem the primary stem, when it is more 

 strongly developed than its lateral shoots, as in most Ferns, Cycadeae, Coniferae, 

 Palms, and Amentaceae. The primary shoot produces lateral shoots of the first 

 order, these again lateral shoots of the second order, and so on. Nevertheless 

 it often happens that lateral shoots of any order become independent, take root, 

 and become detached from the primary shoot ; they then assume all its peculi- 

 arities, and may equally be considered as primary shoots. But it also happens 

 that the primary shoot itself is arrested at an early period, while new orders of 

 shoots proceed from it which gradually become stronger, as in many bulbous and 

 tuberous plants. Shoots which become detached from the mother-plant in a but 

 slightly developed condition, and then continue to grow by independent nourish- 



^ Compare what will be found under Rhizocarpeoe and Angiosperms in Book II. 



