Professor A. Dickson on Development of Bifoliar Spurs. 259 



foliage leaves. A further step in specialisation occurs in 

 Pinus, where, in the adult condition, the leaves on the 

 elongated shoots are reduced to chaffy scales, from whose 

 axils buds are produced, which, after developing from 2-5 

 foliage-leaves, abort at the growing point. In this genus we 

 have then the remarkable condition of the foliage leaves, in 

 the adult plant, being developed exclusively in 2-5-niembered 

 fascicles on shortened and abortive branches, and as these 

 are perfectly comparable to the spurs in the Larch and 

 Cedar, they may likewise be described as spurs developing 

 2-5-membered leaf-fascicles. In Finns sylvestris, and a 

 number of other species, there are only two foliage-leaves 

 in the fascicle, and hence it seems convenient to designate 

 such spurs as bifoliar. These "spurs" in Pinus fall off 

 bodily after a certain period — from two to five years, accord- 

 ing to the species — and, in this, as pointed out by Dr James 

 Stark,* we have an approximation to the condition in many 

 Cupressinete, where the individual leaves are not shed, but 

 wdiere there is from year to year a shedding of leafy twigs, 

 a phenomenon to wliioh the term cladoptosis has been 

 applied. The highest specialisation in branch-development 

 occurring in the order is to be noted in the genera 

 Seiadopitjjs and Phyllocladus, where, in the adult state, there 

 are no foliage-leaves at all, the leaves being all reduced to 

 the form of scales, and where the " leaf- function " is 

 performed by green cladodes which are slender and needle- 

 like in Sciadopifi/s, and in the form of flat expansion in 

 Pliyllodcuhis. It is to be noted that in Pinus, with its de- 

 velopment of foliage-leaves exclusively on abortive spurs in 

 the adult condition, the ordinary or unspecialised condition is 

 exhibited in the seedling plant, where we have a development 

 of foliage-leaves scattered on an elongated shoot — a condition 

 which persists throughout life in such genera as Abies and 

 Picea ; and, similarly, we have in the f^QQiWmg Sciadopitys a 

 few genuine foliage-leaves immediately succeeding the 

 cotyledons, although in later life we have the very highly 

 specialised condition above described, where the foliage- 

 leaves entirely disappear. 



* "On the Shedding of Branches and Leaves of Conifers," by Dr James 

 Stark (with note by Professor Alex. Dickson), Trans. Roy. Soc. Edin., 

 vol. xxvii. pp. 651-59, plate xliv. 



