544 



BOTANY 



TAUT II 



Class II. Ang-iospermae 



/ 



Sub-Class I 

 Dicotylae (^''^ i') 



The Dicotyledons Avith few exceptions possess a pair of seed- 

 leaves ; these on germination either expand as green assimilating 

 leaves or remain within the seed-coat and supply the seedling with 

 the reserve materials stored in their cells. The growing point of the 



stem, lying between the cotyledons, 

 grows into the shoot of the seedling. 

 The main root of the embryo has 

 meanwhile penetrated into the soil ; 

 as a rule it persists as a tap-root and 

 gives rise to a regularly branched 

 root-system. 



The stem has a circle of open 

 vascular bundles, while the root on 

 transverse section shows a regularly 

 3/ alternating arrangement of the 

 ^ xylem and phloem -groups. The 

 ■ meristem situated in the vascular 

 bundles of the stem, or to the inner 

 side of the phloem in the root, soon 

 becomes completed across the medul- 

 lary rays and forms a complete, 

 meristematic ring. By means of this 

 cambium a regular growth in thick- 

 ness of the stem and root takes 

 place (cf. Fig. 147, p. 139). 



The typical form of leaf found 

 among Dicotyledons is provided with 

 a longer or shorter petiole, and often has a pair of stipules developed 

 from the leaf-base ; a leaf-sheath is usually absent. The lamina 

 may be simple or compound ; the latter condition is always the 

 result of branching during the development of the leaf. The 

 margin of the leaf presents considerable variety. The venation is as 

 a rule reticulate (Fig. 51.5). 



The flowers of Dicotyledon.s (cf. p. 489) exhibit a wide range in 

 their appearance and construction. Most frequently they can be 

 derived from a type with five whorls of five members ; increase and 

 decrease of the numlier Ijoth of the whorls and the memliers are met 

 with. The Dicotyledons are divided into the two series of the Ghori- 

 petalae (with free perianth segments) and the Sipupetalae (with the petals 

 coherent). A scries of forms without a corolla are included in the 



Fig. 515. — Leaf of Crataegus with reticulate 

 venation. (^ iiat. sizflj 



