BOTANICAL SUBJECTS. 



147 



The venation of this SxnrcBa is as in Fig. 31. The midrib of 

 the leaflet corresponds to a branchlet, a vein to a secondary branch- 

 let, and a veinlet to a tertiary branchlet, indicating that the teeth 

 of this leaf are abortive secondary and tertiary leaflets. The fascia - 

 tion of the veins reduced the secondary and tertiary leaflets to teeth. 

 Or the reader may reverse the process. If this leaflet were further 

 spread out, the teeth would be liable to become independent leaflets. 



5. A whole branch may be suppressed, and only a scale left to 

 show that it was ever there, or a whole whorl may disappear leaving 

 nothing but alternate scales to show us what happened, as in 

 the Thalictrum leaf. A whole leaflet may be suppressed and leave 

 no trace of its former self, except a minute stipel or gland. 

 Similarly, lobes of leaves may be reduced to teeth, and even dis- 

 appear altogether. In the genus Ilex we have I. aquifolium with 

 very large teeth, or spiny lobes ; in I. crenata and dipyrena we have 

 regular small teeth, and in I. aquifolium Bessoni we have neither 

 teeth nor crenations. 



6. In a Pyrethrum I found that some of the leaflets were opposed, 

 some alternate, and some had two small leaflets at the base of the 

 petiolets, as shown in Fig. 32. This shows that the secondary 



Fig. 32. 

 leaflets («) are in a transition state between leaflets and stipels. 

 Also that in the other leaflets all the secondary sub-divisions have 

 been suppressed. Moreover, it shows that the character of opposite 

 and alternate leaves has little significance, beyond, as I said, the 

 advantage of identifying plants for economical purposes, and 

 studying their possible relationship, as most of these characters 

 are inheritable, and would give indication of community of descent. 



K 2 



