168 



PHYLUM TRACHEOPHYTA: 



tophyte and any female spore develops into the female 

 gametophyte. 



Pteridophytes producing gametophytes outside of 

 spores usually have one of two types of gametophytes, 

 and both types are functionally independent at matu- 

 rity. One type is subterranean, lacks chlorophyll, 

 and obtains most of its nutrients from an association 

 with a Phycomycetes fungus. The other type of 

 gametophyte is aerial, has chlorophyll, and probably 

 obtains most of its nutrients by photosynthesis. Some 

 of these latter gametophytes resemble liverwort or 

 horned liverwort gametophytes. Both gametophyte 

 types normally are less than one inch in total length. 



The pteridophytes, with two types of spores, also 

 have nutritionally independent gametophytes, but the 

 gametophytes are much reduced and are partly to 

 completely within the spores. However, the tendency 

 to decreased size is more pronounced in male game- 

 tophytes; the female gametophytes often are larger 

 than the male. The nutrition in these gametophytes 

 is derived from stored spore nutrients. 



Seed plants have two kinds of spores and their 

 gametophytes are extremely reduced and confined 

 within spores or modified spores. Also, the gameto- 

 phytes complete the tendency toward reduction in 

 size and become entirely parasitic upon the sporo- 

 phyte. In flowering plants the gametophytes are 

 much reduced, sex organs are not developed, and the 

 gametes are produced directly. 



SPOROPH\TE 



The zygote formed from fertilization of an egg by a 

 sperm is the first cell in the sporophyte generation. 

 This zygote develops into an embryo sporophyte. 

 Further development then comes from growth in cer- 

 tain regions that usually are at the tips of root and 

 stem branches or in axillary buds. A bud is a bit of 

 such growth region tissue that is covered, partly to 

 completely, by overlapping leaves. An axillary bud is 

 merely a bud that is in a leaf axil, the small angle 

 formed by a leaf stalk and a stem. 



The adult sporophyte regularly has shoots (or 

 stems), leaves, and roots, but there are no roots pres- 

 ent in known psilopsids. Moreover, one of the two 

 living genera of Psilopsida lacks leaves other than re- 

 duced ones associated with the spore-producing 

 structures (sporangia). All tracheophytes have dis- 

 tinct or much reduced and barely indicated sporangia 

 whose function is to form the spores that grow into a 

 gametophyte. 



ROOTS 



Roots, as opposed to stems, usually are under- 

 ground structures but this is a poor distinction (Fig- 

 ure 11.2). Actually, the fundamental diflTerences be- 

 tween roots and stems are less obvious than mere 

 position. Roots can be distinguished as follows: (1) 

 They function primarily in the absorption of dissolved 

 materials and water from the substrate and in the 

 anchorage of plants. (2) Their apex is a meristem, or 

 region of growth, but this apex is typically covered by 

 a root cap. (3) Root axes are usually naked and with- 

 out superficial appendages other than epidermal out- 

 growths in the form of root hairs. (4) The axes are 

 neither segmented nor jointed by nodes, so there is 

 no root axis separation into nodes and internodes. 

 (5) Root branches originate irregularly and from in- 

 ternal tissues. (6) Vascular tissues usually are not 

 organized into vascular bundles consisting of primary 

 xylem and phloem on alternating radii. (7) An endo- 

 dermis usually surrounds the stele. 



SHOOTS (stems) 



In contrast to roots, stems (Figure 11.2) serve the 

 following'purposes: (1) They are primarily for sup- 

 port, but are generally involved in photosynthesis, 

 storage, and reproduction. (2) Their apices are an 

 uncapped growing region which forms primary stem 

 tissues plus superficial appendages in the form of 

 leaves. The leaves complete development in an 

 orderly sequence toward the growing region and at 

 distinctive dififerentiated legions called nodes. Ad- 

 jacent nodes define an interveining internodal region. 



(3) Branches are present, but they tend to originate 

 regularly from a superficial mass of growing tissue 

 that remains in the axils of leaves (axillary buds). 



(4) Vascular tissues regularly are organized into vas- 

 cular bundles of primary xylem and phloem on the 

 same radii from the stem center. (5) An endodermis 

 may or may not be present. 



LEAVES 



Leaves, the primary food-producing, or photosyn- 

 thetic, organs of most plants, usually are formed and 

 organized at the nodes of shoots. Their arrangement, 

 probably in phylogenetic sequence, can be spiraled, 

 alternate (merely a reduced spiral arrangement), op- 

 posite, or whorled (Figure 11.3). The terms "spi- 

 raled" and "alternate" are often used interchange- 

 ably; however, alternate may be reserved for single 

 leaves at each node so inserted that all leaves follow 



