2092 A TEXTBOOK OF THEORETICAL BOTANY 



wide in distribution, most common in the tropics and rare in Arctic regions. 

 Those occurring in the tropics are mostly epiphytic while the temperate 

 species are mainly terrestrial. 



The Orchid plant may be constructed in several ways. It may be based 

 upon a monopodial construction in which the main axis continues to grow 

 year by year and produces flowers only on lateral branches. Alternatively 

 it may be based upon a sympodial system in which the main axis is com- 

 posed of annual portions of successive axes, each of which begins with 

 scale leaves and terminates with an inflorescence; alternatively the inflores- 

 cences may be produced on lateral axes and the shoot for the current year 

 continues the main axis, stopping short at the end of its growing period, 

 but does not form an inflorescence. 



The terrestrial orchids are all sympodial and usually possess a sliort 

 rhizome, each annual shoot bending up into the leafy branch of the current 

 year. Many form storage organs to tide them over the dormant period of 

 the year. In the majority this takes the form of a thickened internode of 

 the stem, called a pseudo-bulb. In many the bud for the next year's 

 growth is laid down at the base of the stem and from it is developed a thick, 

 fleshy, adventitious root which develops into a tuber which provides a food 

 reserve. 



The epiphytic orchids are either sympodial or more rarely monopodial. 

 The roots of these orchids consist firstly of clinging roots which are insen- 

 sitive to gravity but are negatively phototropic. Secondly, there are absorp- 

 tive roots which may develop between the plant and its support. Such roots 

 may accumulate humus around them and these probably supply nutrient 

 material from the humus which they collect. Thirdly, there are the true 

 aerial roots which hang down in the moist air and are absorptive in function, 

 due to the development of a superficial tissue, the velamen, which acts 

 as a sponge and readily absorbs water. The inner tissues are green and 

 apparently form additional photosynthetic tissue. It should be realized that 

 in the tropics the great majority of orchids drop their leaves in the dry 

 season so that under these conditions the aerial roots form the only remain- 

 ing assimilating tissue. Certain epiphytic orchids which are not provided 

 with pseudo-bulbs persist by the aid of fleshy green leaves. In other respects 

 the vegetative anatomy of the Orchidaceae provides no points of peculiar 

 interest, but reference must be made to the nutrition of the seedling. 

 It has already been pointed out that the seeds are microscopically small, that 

 the embryo (Pig. 2029) at the time of shedding is scarcely developed and 

 that little or no endosperm is present. Under such circumstances it would 

 be virtually impossible for the embryo to survive. In fact, in the absence 

 of the suitable endotrophic fungus it can be shown that death almost 

 inevitably ensues. In the presence of the fungus, however, nutrition is 

 provided for the embryo and its development is made possible. We need 

 not consider here the structural and physiological details for these will be 

 discussed in detail in Volume IV. Nor need we here consider the meta- 

 bolism of the saprophytic species for here again the problem receives 



