DIFFERENTIATION OF ORGANS IN SPERMAPHYTA 15 



Ruscus and other plants to be leaves are so futile as to be hardly 

 worth rescuing from their deserved oblivion. 



fj 



3. The history of development of the stem and of the leaf is usually 

 different. In the first place the duration of development is unlike ; 

 leaves have limited growth, shoots have unlimited growth. But there 

 are many shoots which normally exhibit limited growth, for example, 

 the short shoots or spur-shoots of many conifers and broad-leaved 

 trees ; these however may under certain conditions become shoots of 

 unlimited growth and be transformed into long shoots. There is no 

 real specific difference between spur-shoots and long shoots, the limitation 

 of their developmental capacity is determined only by their position 

 on the tree. We do not know however that this is true of all spur- 

 shoots. It is probable, for example, that the needle-like leafless assimilating 

 short shoots of Asparagus were from the first laid down as shoots of 

 limited development, and that therefore the same reduction in the forma- 

 tion of the organ took place here as we often meet with. Similar features 

 appear in the lower plants. Thus the ' leaves ' of Chara are merely 

 short shoots, but it is impossible, so far as we at present know, to cause 

 them to develop into long shoots, and it is extremely improbable that 

 this will take place. Again, no one has succeeded in forcing artificially 

 a leaf of one of the Spermaphyta to unlimited development. Nature 

 however sometimes tries this experiment. The leaves of some ferns 

 continue to grow at their points in successive periods of vegetation. But 

 a much more striking illustration is furnished by species of the genus 

 Utricularia, which are amongst the most remarkable plants in the world. 

 In this genus the floating ' shoots ' of the water-form, as well as the 

 creeping 'stolons' of the land-form, are, as I have proved 1 , homologous 

 with leaves ; but the difference between stem and leaf has entirely 

 disappeared. The organs which are homologous with leaves produce 

 flowers and other shoots, and exhibit unlimited growth ; and that they 

 are really leaves with prolonged apical growth is only to be determined 

 by a careful comparative study. Every distinction then that we may 

 draw between shoot and leaf is only relative, is not fundamental. The 

 method in which the leaves are laid down at the vegetative point of the 

 shoot-axes is not fundamentally different from that exhibited by the 

 shoots, and no advantage would be gained by discussing here the question 

 of the degree in which the several cell-layers of a vegetative point share 

 in the primordium of leaf or of shoot. There is however this point still 

 to notice leaves are in most cases outgrowths of shoot-axes, and they 

 arise on their vegetative point as lateral members ; nevertheless terminal 



1 Goebel, Der Aufbau von Utricularia, in Flora, 1889, p. 291. More details will be found in 

 Morph. u. biolog. Studien, in Ann. du Jardin Botanique cle Rnitenzorg, vol. ix. p. 2. 



