Miscellaneous. 340 [Oct. 1906. 



So 



Lessons in Elementary Botany. V. 



By J. C. Willis. 



We must now pass on to consider the reproductive organs, remembering that 

 the chief feature in the life history of a plant is its reproduction. It is customary to 

 distinguish between vegetative and true methods of reproduction. The former is the 

 detachment of portions of the vegetative system— specialised for the purpose as in the 

 case of the tubers of a potato, or not, as in the case of rubber cuttings— which may 

 grow into new plants without any further reproductive phenomena, s True repro- 

 duction, on the other hand, is propagation by special cells (the unit parts of plants) 

 set apart for the purpose. These may be able to grow into new organisms without 

 any sexual process, or they may require, as a preliminary to further growth, to fuse 

 together in pairs, male and female. In the first case we speak of asexual reproduc- 

 tive cells or spores ; in the second of sexual, or gametes, which by their union produce 

 a new cell or zygote capable of further development into a new individual. 



The spores are contained in little capsules or sporangia seen in their typical 

 form on the back of a ripe fern-leaf, where they are usually aggregated into groups 

 called sori. As a rule in ferns the sori may occur on the backs of any or all of the 

 leaves ; but in many cases they are on special, usually smaller, leaves — usually with 

 narrow segments. In the clubmosses, near relatives of the ferns, the sporophylls, as 

 the spore-bearing leaves are generally termed, are aggregated into cones at the end 

 of the branches, and a somewhat similar phenomenon is seen in all the flowering 

 plants, the flower corresponding (to a large extent) to a cone, the stamens and carpels 

 being the sporophylls. 



In the ferns there is only one kind of spore, all being of the same size, but in 

 the selaginellas (common in upcountry jungles, creeping on the ground) there are 

 large and small spores ; and the same is the case in other plants. 



When the spores of ferns are sown upon damp soil (try this on wet sand) 

 they germinate and give rise to delicate little green leaf-like plants, called prothalli, 

 entirely different in structure from the ordinary fern plant. The prothallus is 

 sexual and bears the male and female cells or gametes, which when united give a 

 zygote capable of growing, not into a prothallus again but into a leafy fern plant 

 like the one with which we started. The female cells remain in their receptacles on 

 the prothallus, aud the new plant is therefore attached to the prothallus for some 

 time (allow the fern spores to grow for a considerable time and presently the young 

 fern plants will appear on the prothalli.) We may therefore sum up the life history 

 thus :— i 



Fern plant > sporophylls > sporangia > spores 



A 



V 



< male gamete < male receptacles) ^ i, 



Zygote < female do. < female do. jProtnallus. 



In Selaginella, &c, the big spore gives a female, the small a male prothallus. 



In the flowering plants this same alternation goes on, but the prothallus stage 



is much reduced. The big spore germinates without falling off, and the small spore, 



or pollen grain, is carried to its neighbourhood by the wind or by insects, and 



germinates there. The growth of the zygote stops for a while, after a time, and we 



say that the seed is ripe. 



Before we consider the flower in detail, we must briefly describe the ways 



in which flowers are arranged upon the plant— the inflorescence, as it is termed. 



Very commonly a plant has only one inflorescence or tuft of flowers, but most often 



has several. 



