208 G.ORSHAN 



THE LIFE CYCLE OF SELECTED CHAMAEPHYTES 

 Practically all chamaephytes in Israel are seasonally dimorphic but the 

 seasonal dimorphism is not always achieved in the same way. Two types 

 of branches may be distinguished in most of them, i.e. dohchoblasts and 

 brachyblasts, but these terms may be misleading as the same branch may 

 start growing as a brachyblast, remain short for a considerable time and 

 later start elongating and turn into a doHchoblast. The elongation generally 

 takes place in spring which is the main growing season when both tempera- 

 ture and water supply are optimal. Such a brachyblast may be called a 

 temporary brachyblast. In other plants the dohchoblasts and the brachy- 

 blasts are not merely different stages of development of the same branch 

 but are developed from different buds and a brachyblast never turns into a 

 doHchoblast. This type of brachyblast may be called a permanent brachy- 

 blast. Moreover, the brachyblasts may also be classified according to their 

 degree of elongation, absolute brachyblasts which do not elongate at all 

 and resemble small rosettes, and partial brachyblasts which show a 

 certain although hmited degree of elongation. 



The dohchoblasts and brachyblasts are generally associated with two 

 leaf types, i.e. larger and more differentiated winter and spring leaves, and 

 smaller summer leaves. The seasonal dimorphism is attained by the 

 shedding and growth of the different types of branches and leaves at 

 different seasons. 



A few types of chamaephytes differing in their types of branches and the 

 sequence of their development will now be discussed. 



(a) Thymus capitatus. The dohchoblasts start elongating in January from 

 temporary absolute brachyblasts developed in the preceding spring. In 

 February they attain a length of 1-2 cm. During March and April, their 

 growth rate increases and branching takes place in their upper portions. 

 At the apex of these branches the inflorescences are developed during June 

 and July and the plant sets fruit in September. These dohchoblasts bear 

 larger leaves which are gradually shed in an acropetal direction, from the 

 middle of May until August. From the buds at the axils of these leaves 

 brachyblasts bearing small scale-hke summer leaves are developed. They 

 remain short during the whole summer and only some of them, mainly 

 those on the lower portion of the dohchoblasts, start to elongate in winter 

 turning into the dohchoblasts of the next season. The others which do not 

 elongate, subsequently die and fall off. The upper portions of the doHcho- 

 blast die back and are also shed (Fig. i). 



(b) Teucrium polinm. The hfe cycle of this plant resembles that of Thymus 

 but its brachyblasts are partial brachyblasts elongating slowly during the 



