1652 



A TEXTBOOK OF THEORETICAL BOTANY 



selected for making mathematical rules and drawing instruments and is a 

 favourite with wood carvers. In the Prunoideae gum is often produced by 

 the disorganization of the wood, probably due to bacterial infection. 

 Prickles and other hard epidermal outgrowths are common in certain 

 genera (Rosa, Rubus), and branch thorns are produced commonly in others 

 {Crataegus). 



The subdivision of the Rosaceae as proposed by Focke in the " Pflanzen- 

 familien " is somewhat elaborate but in following it out carefully we may 

 find a method of separating what, at first sight, appear to be closely similar 

 genera. 



I. Spiraeoideae 



In this sub-family there are from one to twelve, usually two to five 

 carpels, which are arranged in a whorl, being neither on special carpo- 

 phores nor sunk in a receptacle. Each carpel contains two or more ovules 

 and the fruit is usually dehiscent, frequently being a follicle. The flowers 

 have a pentamerous calyx and corolla. The stamens vary from ten to 

 indefinite. The plants are usually shrubs, devoid of thorns or prickles, 

 with simple or compound exstipulate leaves. This sub-family is considered 

 to be most closely related to the Saxifragaceae. It is divided as follows: 



I. Spiraeeae. The fruit is a follicle and the seeds are not winged. 

 Spiraea (Fig. 15 13). 



Fig. 15 13. — Spiraea japonica. Inflorescence. 



2. Qiiillajeae. The fruit is a follicle but the seeds are provided with 



wings. Oiiillaja, Lindleya. 



3. Holodisceae. The fruit is an akene. Holodiscus. 



II. Pomoldeae 



In this sub-family the floral axis forms a deep cup with the carpels more 

 or less completely united to its inner wall, and with each other. The carpels 



